I have a '92 3000GT VR-4 with a cold engine stumbling problem. The engine starts perfectly hot and cold, but while cold stumbles badly when accelerating through about 2k RPM, with occasional mild backfire in the exhaust. The problem mostly disappears when the turbos kick in, and after warmup, but I can still feel an occasional "hiccup" in the engine while running at constant speed after warmup.

It has 60k miles on it, and I just had the 60k maintenance done, with new timing belt, water pump, fluids, fuel filter, and plugs/plug wires. The problem existed before the service, and I was hoping the tuneup would cure the problem, but it had no effect.

The computer has no codes stored to indicate a problem. The Check Engine light stays off. The air filter is old but still fairly clean. I'm having my mechanic checking fuel pressure this afternoon; if it's not spark, it might be fuel.