Hi, I have a 1991 GP that has caused some problems over the last few weeks. It started with a violent hesitation when I accelerated, and would quickly die while in gear. I had no problems driving the car for around 5,000 miles before all this happened. Since then, we've replaced the spark plugs, wires, pcv values and related hoses, sealed other hoses to fix vacuum leaks, fuel filter, air filter, crank sensor, and knock sensor.
Start-up is good. Runs fine in closed loop with about 14 1/2 in. on vacuum gauge. Closed loop there is a small sputter, but everything else is fine. Increase RPM to about 1500-2000 and vacuum snaps to 19 in. and then returns to normal. Runs good at high RPM for about 8-10 seconds, then vacuum goes to 0-2 in. and the engine spits and sputters. 8-10 seconds later and all is normal again. This cycle keeps repeating. No codes in ECM. All other items check out fine. Fuel pressure is steady at about 41 psi.
Does anybody have an idea what it could be?