Car drives great until it's fully warmed up and then it loses power at higher RPM's and problem is more severe under heavier load, like climbing steep grade. After doing some initial diagnostics myself (non-contact thermometer, vacuum testing, and removing O2 sensors), I had it flow tested a couple weeks ago to confirm excessive back-pressure and the restriction was said to be in the underfloor cats.
A couple days ago, I replaced the underfloor cats with just pipes. After a few miles, though, the problem reappeared. It seems to be driving better than before but, once it reaches normal temp, and I either open up the throttle or encounter a steep incline, it starts to lose power and surge, almost bucking, until I ease off the throttle and let it drop down into lower RPM's. It seems to occur only over 2000 RPM's.
So, yesterday, I removed the rear mufflers and resonators, thinking some chunks from the cats must've broken loose and drifted down the pipe. Took it for a test spin and issue showed up right as expected. I pulled over and removed the air filter to climb a hill, but no improvement.
I just did another vacuum test. Unplugged the EGR and it was holding steady at 17.5 at idle. When I'd raise the RPM's to 2500, it would dip for a split second and then return to 17.5 and stay there. I'd hold it a 2500 for a min or so and no change. When I'd release the throttle, it'd jump up for a split second and then settle right back at 17.5. So, that seems normal and good to me.

I don't know what to check from here.
I'd gladly welcome any suggestions. I do have a scanner. If providing any info from that would help, just let me know what to post.
I drove it up from FL (to NC) a few months ago and it drove great - no issues and had passing power to spare. Was having fun on these mountain roads but now...not so much. Was hoping to drive it to TX for the month of March but, if I can't do it at Interstate speed, I'll need to make other arrangements.

I don't mind getting my hands dirty and am a pretty quick study, but I've reached the limit of my current diagnostic skills.

Please help!

 

Do you know how old the fuel filter is? I had similar symptoms --
when accelerating - lurch / jump / hesitate .

I found on the forum that it could be the fuel filter. I completely forgot my fuel filter for the entire eleven years I have owned it.

It was simple enough - but I have a hydraulic jack and jack stands.

I knew it would be an improvement when I got the old filter off and could not blow though it, compared to free flowing air when blowing through the new one (use some adapter so your lips don't actually touch).

Solved the hesitation / jumping / lurching.

 

I was thinking you have a fueling problem, as well, while reading. A bit of mystery why it would work fabulously until it comes up to operating temp, though.....

 

I know sometimes it can be a problem when certain fuel pumps reach operating temperatures that they tend to lose pressure before they're about to die. It may be a good idea to run a test on your fuel pressure. It may not be getting enough fuel once it's warm,causing the lack of power.

 

This has many of the hallmarks of a fuel starvation issue. When cold, the mixture is being enriched, and it could be this which is masking the problem until the engine is warm.
Fuel filter is a good place to start if you haven't been in the habit of changing it, and the fuel pressure regulator could be another interesting port of call, as has been suggested. Could be a dodgy fuel pump, but I would try to eliminate the other potential causes first.

 

Thanks for the replies, fellas.

So, I changed the fuel filter awhile ago, with no improvement. Since then, I've put new could and plugs but the problem is still there.

I've submitted a new thread this am with updated info titled "Power Loss/Surge Under Load When Warm - Erratic RPMs on Scanner Graph but Tach Steady" or something pretty close to that. So, I'll spare everyone the reposting.