92 cutlass cutting out for fractions of second

1992 Ciera with 160K miles. At any speed, speedometer will drop down to zero, check engine light, temp light flash briefly. Engine feels like it is stopping. Then everything comes back to normal. Happens lots when cold. Other symptoms, with engine not running, turn ignition to run and speedometer jumps off zero (as high as 40mph) then drops back to zero. Engine not even running. Have disconnected speed sensor. Problem at speed still occurs but without speedometer moving (at zero). Thinking ECM replacement. Any other suggestions?

Make sure all the electrical connections from the relay casing , fuse block and to the altenator are all as tight as can be.

could be a bad alternator causeing the drivability problems. if the voltage regulator is bad it can cause thoes symptoms. also a bad alternator shorted out bad enough can kill your dash cluster, i had one go out in a beretta i was working on, thought the dash cluster just died, but about 15 min later the alt was dead.... car died on highway.... not fun. Take the alternator to any Autozone and they can bench test the thing for free. as for the dash cluster if its bad..... junk yard

Before I got a chance to check anything the car has died. While driving, system cut out but this time speedometer went to maximum. Car will not restart. There is no service engine soon light at all anymore. Tried to bring up codes and nothing happens with jumper across diagnostic terminals. Car appears brain dead. I replaced ECM ($80) but no difference. But did not realize new ECM did not come with PROM. Had to take PROM from old ECM and place in new ECM. Looks like may be bad PROM. Can anyone provide guidence before I spend more money on parts that may not get car running again?