Can't go over 50 mph

[91centurycus]

Hi, I've had my 1991 Buick Century Custom 3.3 for about a year now. On two past occasions I've had problems. The first time I couldn't accelerate past a certain speed and it shuddered a lot. My mechanic replaced the computer chip and did some other things and then it ran fine. About a month ago it started dying a lot, when I was in idle, switching gears, etc. The mechanic didn't do much, just cleaned some stuff and for the last three days it seemed to run great. But yesterday it suddenly acted up again. I can only press the accelerator so far, when it reaches a point it will no longer accelerate. So I can't go over 50 mph. The service engine light came on for about a minute. Then it died, and when I turned it back on and in drive, the thing would go forward without me pushing the acc. I think it has something to do with the computer. Any thoughts appreciated.