1988 Oldsmobile Cutlass Stalling worsens the longer the car

We just bought this car, 88 Olds Cutlass. The owner told us that after long trips (1 hour+) it would stall when coming to a stop, but start back up instantly. After 6months of this, it got worse and stalled 10 times within a 10mile trip, each time starting right back up. A mechanic shop replaces one of the sensors that deals with the cooling system. This didnt fix the problem. I purchased the car assuming a simple fix.

Since owning the car, the stalling has gotten worse. We have noticed that the longer the car is running, the worse the situation gets. Although for the first 20-30min of use, the car runs near perfectly. Changing from periodic stalling at stoplights to surging while driving, bucking, and stalling everytime the car is not accelerating. Also, the check engine light would come on and off while this was happening. Gauges would also bounce around.

The IAC valve would not move when the key was turned on, but would move when key was turned to off. We replaced this, the car seemed to run better at first, but the problem seemed to actually get worse with the new valve installed.

Our only assumptions now is that the computer is sending erratic signals out. The longer the car runs for, the more erratic the operation of the vehicle gets. We do not have access to computer testing equipment, and testing it is very hard as the car has to be put through use for an hour or more before problem becomes fully apparent.