1996 Geo Tracker - Engine will shut off sometimes while sitting and somteimes while driving.

Sometimes when car is put in reverse the engine shuts off. Sometimes engine misses when brakes are applied. Tach goes to zero almost quicker than engine shuts off. Replaced mass air, crank sensor, PCM and distributor twice. Checked fuel pressure, grounds and wiring, throttle position, coolant sensor. Some days the car runs fine for a little while. It ussually restarts fine. Only code it had was for the nuetral safety switch saying it did not know gear position. I have started the car and unhooked switch and it still would shut engine off. I am running out of ideas.