I am having the same problem with my 93 GT for several weeks now. Only modifications are a K&N air filter, UPR catted X pipe, and Flowmaster 40's. I recently changed my distributor cap and rotor, and replaced my spark plugs(autolites) and wires(MSD). This past weekend I replaced the fuel filter, and checked the fuel pressure which was at 40(WOT), and the timing is at 10 degrees. But the car is still hesitating and loosing power at around 3000 RPM's at WOT

 

Well ,bought some bosch 4+ plat plugs and the car ran waaay worse.Double checked everything but around 3k the car would bog badly and just not go.Put the NGK's back in and was back to original high rpm stutter.This morning a new problem arised.When I started the car the rpm went right to 2k and stuck.It was like this until i shut it down and let it sit all day.After work she started fine.I stopped at a friends and was there maybe ten minutes, came back out and car was right back at 2k rpm.Turned off car for a few at the store.Started it and back to 2k.Pulled out, hit the gas a bit and it went back to normal rpm 800 or so.I have double, triple checked EVERYTHING. vacuum hoses, wires, i just dont know what is happening to my 5.0!!!!

 

Maybe your high rev miss is floating valves from worn springs. I have the same idle issue from time to time...when it is doing it, unplug you IAV, if it drops down try and clean that out with carb cleaner..I don't know???

 

Platinum plugs and TFI don't match....the idle problem you're having now, could be caused by the TPS or IAC...you might want to run a set of DIY KOEO/KOER self-tests to check what the EEC-IV is working with...+ do a base idle reset, clean TB, IAC & MAF and check TPS voltage. My .

 

Well its back to idling fine(ghost in the machine I guess).Still popping at high rpm.No MAF this is a speed density setup but is a 93 5.0 motor.I am getting 32lbs fuel pressure at idle and 42lbs at WOT. Are these good numbers or am I not getting enough fuel? Maybe the fuel pressure regulator is going bad? Going to change it out for an adjustable FPR.Also going to change out MSD coil and buy a new Distributor.If none of those fix the problem I guess its off to the shop where someone can hook it up to a computer and tell me what the hell is wrong. Thanks again guys.