1990 Oldsmobile Cutlass Stalls, Hard starts, Surging (once)

My very reliable car suddenly stalled one day after driving it several blocks. It restarted and drove another block and then stalled repeatedly at a stop sign. It took several attempts to get it to run long enough to move. A week later, stalled and wouldn't restart when I pulled up to a gas pump. The mechanic there had no suggestions. It restarted after a few tries, and drove and restarted fine for most of the over 100 miles I had to go that day. Then the service engine soon light came on while driving in traffic at about 50 mph, and the accelerator felt "funny, " as if it would not respond if I had been able to hit the gas, and the speedometer suddenly dropped to 20 mph, though I was at the same speed. Engine light remained on and speedometer stayed at 20, until traffic cleared and I had the sense that I should just hit the gas, which I did, and the engine light went off and speedometer returned to normal. I had to make one stop, and it did the stall/ hard start routine, and then, when it did restart, it surged as though I had floored it (though I wasn't even touching the pedal). I got it home, and it has been treated to fuel injector cleaner and gas additive, and now won't turn over.