Hmmm. I think this may be MY problem. Here's what happened on my cross country drive:

First day: hot, ended with a lot of driving between 80,90+ MPH. Faster than I do in LA. Took an offramp, but didn't need to come to a full stop, as I was merging onto an empty road. went down to about 10MPH, and when I started to accelerate, the car began lurching, in that way you did when you were learning how to drive and you popped the clutch. It was pretty intense. I pulled over, put it in neutral, revved it a little, pulled out slowly. Better, but still a little lurch-y.

Next day: a little lurch-y on cold start but that seemed to go away. my car passed the 50,000 mile mark in the middle of nowhere in Utah, so to celebrate I REALLY wound it out. Between 120-140MPH. Somewhere in there the check engine light went on. I was kind of paniced as I was out of cell range. When I got close to a city I called my jag tech in LA, who said he was "90 percent certain it was emissions-related," and that "three cold starts" and the light would be off.

Which is exactly what happened the next morning. Lurchy again at cold startup, but better. About four hours in (again, after a period of fast highway driving), check engine light came on. Interestingly, it was off again on its own by the end of that day. Has not come on since. (This was a few weeks ago)

One thing that I'm sure is transmission: by the end of the drive, had downshifts on acceleration at highway speeds (say, to pass someone at about 50-60 MPH) became harsh, almost violent, like a THUNK when the car would SLAM into the lower gear. You could almost feel a harsh kind of punch.

Made it the east coast, and I haven't driven the car for more than 30 miles at a time since then.

I notice the car idles a little rougher than usual now at cold start-up, but that goes away. Shifting still a little lurchy at cold start, which gets better. But other than those issues when cold, if you gentleman-drive the car, and weren't familiar with it, you wouldn't notice anything other than the car doesn't seem to like to downshift, and it lug-lug-lugs a little if you hit the gas and are at like 1400 rpm.

One last symptom (that may be unrelated but what the hell): There is now a faint-but-noticeable sound that sounds like pre-ignition on hard acceleration. I'm running 93 octane Shell in the car so I'm sure that's not the case, and it's a sound I haven't heard in decades, but I do remember the sound. This is a lot like it, only faint. And if you don't push it you don't hear it at all. Someone suggested it might be the injectors?

Could this all be related to the cats? (And am I dealing with more than one thing here?)

 

That sound may be chunks of cat rattling around.

 

Easy to eliminate the suspicion.
Run 3 bottles of this (one at a time)

Identical chemistry to the BG44 service I believe you had done. If you see an improvement you will know what to do.
There is no downside, your car needs it. You will have to change the oil afterwards.

 

This car is going to drive me to drink.

Floored it on the on-ramp this AM, went into restricted performance mode, no CEL. After a few miles the CEL came on and the exhaust note changed a bit, now the idle sucks, like it's 200 rpm too low or something. Very jerky from a stop, but it doesn't die. My el-cheapo code reader doesn't see any codes.

If it wasn't so damn pretty this thing would be gone by now.

 

You clearly have one or more codes so the problem is with the tool not reading them. Start there.

Bad idle may well be an air leak in which case look at the many threads about fuel trims.

 

I'm fumbling around in SDD right now, so far the main DTC is cylinder #4 missfire.

Crappy pic, here are the codes: