92chevy Camaro rs 150K stalling, hard start, bouncy tach

92 chevy camaro rs V6 3.1, 150,000 miles
it all started with the tach(rpm) bouncing a couple hundred as I was driving home on the interstate. A couple days later it started stalling at idle or when I pulled up to stop signs and sat, it would die. The rpm needle jumps a lot while idling before it dies. Then it became extremely hard to start and had pump the gas pedal to get it to start. But will start eventually and run if I keep my foot on the pedal even when stopping. The things I have replaced are the spark plugs, oxygen sensor, idle air control valve, MAP sensor, throttle position sensor (TPS), ignition coil, fuel filter, fuel pump. No vaccuum leaks that I have found. Does not have a mass air flow sensor(MAF). Have checked the timing and is fine. No check engine light except when it stalls it gives a code 33 which is the map sensor system. I've checked the fuel injectors but the reading doesn't make sense because it zeros out just as if I crossed the prongs on the ohm meter. I also checked a brand new fuel injector from a parts store and got the same reading. The parts store guy got the same thing. Please help I am at a loss as to what else could be wrong!