3.8L EEC-IV, TBI (CFI), C6 Tranny

This engine is actually in my '86 Bird, but I've been having this problem for a very long time. I had to have a long block put in about 2 years ago, thanks to some 'expert' mechanic leaving a shop rag under the intake when I had intake/timing cover gaskets put on it, but the problem was happening even before that, so I know it's not anything below the intake. It's gotta be electronic, but darned if I can isolate it.

I've checked everything I can think of and get NO codes unless I unplug something and generate one, which tells me the EEC's working pretty good. But just to get a few more scratching their brains, I'm gonna describe it.

When the engine's cold, it runs great. On the highway, it runs great. However, get off the highway, and drive about 4 or 5 miles, and it starts acting like it's running out of fuel. Same deal if just buzzing around in the city. Stop at a light, and it'll nose over when starting off. It doesn't surge or idle rough at all. It'll take off like a bandit at WOT. It's just when the accelerator is used normally for driving. Ok, I know everyone's going to point at the TPS. Will a TPS fail in such a manner that it has a problem only at a certain temperature? I think the car is running cool enough that it's having a tough time deciding whether to run in open loop or closed loop, and it's the switching back and forth that's making it do this. I thought about changing the t'stat out to a 190 or 195, but I don't even know what's in it now. I'm assuming the dealer re-used what was in it when they used my old intake on the new block.

This problem is completely random. Some days, it doesn't happen at all, others, well, let's just say that it's frustrating to the point of HATING a car that I fell in love with in 1984, and had to have one in 1986. I'm reasonably certain it's temperature related, because even if it's doing this when I get on the interstate, it goes away very quickly. The temperature goes up quite a bit on the interstate, but never really gets hot to the point of concern. I don't think I've ever seen the gauge above mid scale, except two times. Once was when the instrument voltage regulator went out on the back of the cluster. That was pretty obvious when ever gauge on it pegged out. The next was a clogged radiator which I replaced. The ex was not very kind to this car which I bought for her brand spankin new. I think it was more a case of barely knowing where the gas cap was, and not knowing why steam would come from under the hood when the little needle on the temp gauge was no longer visible (blown heater hose).

The new wife is driving the old car with the new engine. I love the old bird because it drives like it's on rails. I just wanna get rid of that nuisance problem. It'll become my daily when I get a um er eh mini umm, mini van. There, I said it. I've got too much invested in the old bird to replace it right now, especially for a work vehicle on which I will average about 40K per year. Besides that, like I said, I love driving it. It gets great mileage for one with no OD, which I may help out by putting an AOD in it one day. Too old for electronic tranny, and I ain't interested in re-wiring the car.

Any other thoughts on what this might be? Gonna be a fun one to fix, 'cause the dealers couldn't do it, nor any other mechanic that's tried. Then again, the last mechanic that tried was the one that left the shop towel under the intake manifold. I also forgot to mention the same mechanic set the timing on it to 10 deg ATDC. DUUUUHHHHH.