Video attached! 2007 XK.

New battery. Haven't tested it, but sits at 14.1-14.4 while running. I have a new multimeter coming in the mail at almost the perfect timing. Replaced it initially because my music kept being funky. That seems to have cleared itself up.
Now - for the meat and potatoes. My RPMs were already pretty fluttery ( +-75rpm give or take) for the first 1-3 minutes of being turned on, depending on the temperature outside. 2-4 Minute window being how long it 'warms up' for. If its a hot summer day, it is not going to take 4 minutes to get comfortable. Wintertime, sure. RPMs fluttered the whole time though regardless of the season, just during that warmup period. After that, it would idle perfectly and nothing could shake it.
This morning, I start the car and sit there as I wait for it to get to temp. Reverse, back out of my lot - and I head down the road. Pretty much as soon as I hit 20-25MPH, all of these faults pop up on the cluster. Engine fault, Park fault, DTC fault, CATS fault - I assume people have these faults appear when one of the systems break. I have yet to see any of them - I have yet to even see a check engine light. But here I am with every fault you can imagine!
Car goes into limp mode, wont rev past 1250RPM or so, but it allows me to get back to my lot - which was cool. I was only 250ft or so down the road. Shut the car off. Didn't get out of it. Kind of just shaking my head and coming to terms with having to deal with this - I eventually started the car again. Starts, revs itself to about 1750ish, and is now fluttering RPMs in a window of about 150-225 RPM. In the video I attached you can actually see it fluttering, but it was a bit worse in the beginning. I have the ODB2 port being used by a little information screen tool installed next to my A Pillar showing me time, miles, outside temp, MPH, etc. I have not tried to turn the car on today or do anything else with it at the time of this post. I am about to go outside and unplug the ODB2 tool, and plugin my Foxwell NT650Elite to see if the faults tell me anything. I don't see any check engine light - not sure if faults would tell me anything... But gotta have these experiences. Ready to jack the car up and get as dirty as I need.

Mentally the battery shouldn't be in the issue threshold - but knowing how these things are... not even totally sure at the moment if the battery is topped off or what the % on it would be - but I do drive the car everyday. Wondering if it is an electronic issue that does indeed relate to the fluctuation of RPMs that ended up finally snapping. I absolutely grandma the car and never hotrod it, as much as I want to. Maintenance is kept up super well & have spark plugs + ignition coils ready to be installed when the day calls.
Thank you for any insight and/or tips on where to go from here, or ideas on what could cause this!

 

Just because your battery is new doesn't mean that it's fully charged. New batteries often sit on retailers' shelves for months discharging before they're sold. It's best to put a brand new battery on a battery charger (10 Amps or more) and make sure it's fully charged before you install it.

Even though you drive it every day, it might not be for long enough or far enough for the alternator to fully charge the battery. You should have the stored OBDII DTCs (Diagnostic Trouble Codes) checked for possible alternator problems.

Never leave a dongle plugged into your OBDII port after you shut off the engine. Doing so can light up a Christmas tree on your dashboard.

 

SNAPCHAT, since it is an across the board (all cylinders) I would start to zero in on a problem of low fuel pressure or less likely, poor temperature reading.
The unit that regulates the pressure to the fuel rail could be running at too low of a pressure. Could be one of possibly 2 temp sensors. To keep the costs down Ford parts could work or aftermarket.

 

Only as there is no mention of it, reading the voltage while the car is running is no test of battery voltage.
As per the workshop manual, there are a few pinpoint tests that must be completed. Confirming battery health number 1. Collecting relevant DTCs, number 2.

We need the codes AFTER you confirm the battery health.

 

Have the battery hooked up to a charger right now. Gonna let that go for a while. Did not attempt to start car again, but did pull codes before I took the battery out. My device gave me these codes.100k miles - could the cats being clogged/clogged just enough, cause a misfire? Does the first code exist purely because it went into limp mode?

 

Battery fully charged over the course of about 17 hours on a 3a charger. Smart charger shows battery health - says 99%. Still cannot take a sitting reading on the voltage - will post that as I go in about a day. Installed the battery back into car - I have 2 videos attached of the car during a cold and a warm idle. On its first start, the RPMs actually fluttered a lot more than it usually would - which was weird. I cleared the codes with the scanner the last time I was in the car. The car eventually got warm, and idled at its regular numbers, smoothly and normal. Not sure if that is flabbergasting to anyone - kind of reinforces the temperature idea, in my head. Drove it up and down the block and revved it a bit to try and stir something up - nothing yet. Sort of acting normal, albeit when cold again will have another fit.

 

Warm idle seems a bit high. Rough cold idle. It might be worth it to change the temp sensor in the thermostatic housing.
In the mean time some codes may reappear.

 

Part en route - already have a thermostat sitting on the shelf waiting to go in. Probably will need the housing too since a lot of em break, but I will wait on that. Going to do my spark plugs and ignition coils while I am in there. I will report back what is happening when all of this is done. Thank you.

 

Disconnecting the battery clears learned values (not codes) such as fuel trims so it has to relearn. It will take longer than one warm up but is probably nearly there already. Just drive it normally for a bit then check the fuel trims in the usual way.

You've also cleared all the OBD monitors (codes clear and again with battery off) and until they all set again many codes can't flag. You can see status of each OBD monitor using OBD.

 

I had a similar issue with the RPMs fluttering at idle, and for me, the problem was a vacuum hose connected to the fuel pressure sensor/regulator. The plastic piece on it was broken, and someone had poorly glued it.

 

It has been about a week - battery was fully charged as per my posting^ and the issue has not returned. I will blame the battery for now.

 

You are not alone. Many jump to complicated assumptions before adequately/correctly completing Pinpoint Test #1.
I appreciate your feedback.
A lesson learned.