1998 Chevy Cavalier surges and stalling

My Cavalier started dying on my a couple months ago, it acted like it ran dry and normally ran fine in cooler weather, but in warmer it would act up. My repair center replaced the O2 sensor and sent me home with it, but I noticed a "clicking" noise, and it died in the driveway. So I took it back. It drove over there fine. The repairman took everything apart and couldn't determine what the problem was. He said it needed a new computer. I couldn't afford it at the time, so I was determined to take the car home and store it until I could have it repaired. Well, now it surges so badly that the motor heats up. Also, when you drive it, it is uncontrollable, you have to ride the brakes to get down the road, and when you pull into a parking lot, it dies. The repairman says it's just the computer "getting worse" so now it's sitting on his lot, un-drivable. I think he messed it up worse than it was, but he denies doing anything incorrectly. Do I have to replace the whole computer at $500+? He says even if we find one, we have to take it to a Chevy dealer to have it "re-programmed" to my car, which will cost about $200.
help?