Uh oh!! Remember a thread I had last summer where my parents were on their way to Missouri in their old 1992 Park Avenue Ultra and they were having problems where the longer they drove the less throttle they could give because the engine would bog? They stopped and checked codes (no codes). Put in a new fuel filter (didn't help). It kept getting worse. Finally they had it towed in to a Chevy dealer. Discovered the dreaded 1992 gas tank baffles problem. The baffles were falling apart and were blocking fuel from the pump. New tank and pump and there was no sign of fuel starvation at all. They put another 10k on the car before selling it.
Just remembered this and thought you might drop the tank if you have the time and tools. Pick up a used $50 tank and pump from a 1994/99 in a salvage yard, and put a new strainer on. Hopefully this will solve it. Not saying it is the problem, but just a little experience I've had. I had the same problem on my first Bonneville (1992 SE) and finally the pump died. Put a tank w/pump out of 1994 Lesabre in and never had another problem.

A new tank!?! Crap.

But why would it get better after the car cools off?

I've been watching this one carefully, and the FPR makes the most sense to me. There isn't alot more that can cause that fuel-wise. Your fuel pump is almost always a constant temp, cooled by the fuel. Fuel filter won't be affected by the heat, and all the injectors wouldn't do this at once.

The FPR, however, can be affected by heat assuming an advanced age. If that ain't it, I'm stumped.

You read my mind, Bill.

no updated FPR yet?

It doesn't look like it'll fit. Too short and narrow at the tip. Any advice?

Finally got a new FPR put in on Friday. My problem still exists, but not as prominent. I'm beginning to suspect I don't have enough shielding on the intake, or my chip doesn't agree with the mods.

That'* not entirely true.

There are two fuel lines between the tank and the motor. One for fuel delivery, and one for excess fuel return. As the motor heats up, the fuel that'* going back to the tank gets warm as well. As an automotive tech, I have seen this issue before (bad fuel pump after vehicle has warmed up). Experience is the most valuable teacher of all...

CSFiend is right on the money. When I had my 1994 SLE up at BIR last year and the pump was dying out I had to continuously ease up on the pedal because the pump was heating up at a rate that required constant easing up. It was also somewhat submerged in fuel (3/4 tank).