Transmission or a computer problem?

1995 Saturn SL2 (automatic) with 230,000 miles
I started having what I thought was transmission problems 7-8 months ago. As I would be making a left hand turn, the car would drop into a lower gear than normal. It would refuse to shift into a higher gear until 30-45 seconds later. Maintaining the current speed or pulling over to the side of the road for a minute would 'clear' the problem and the car would then shift normally. This was very rare in the beginning, maybe once a month (but it progressively got worse). The car continued downshifting while making left turns and if it happened more than once during the same trip, the check engine light would come on. I had a lube shop plug it into their computers and they said they didn't see anything other than a few really high numbered codes that cleared after a few cycles.

I work about 45min away and this downshifting starting happening 3-4 times per trip and is now happening when slowing down or speeding up. I usually wait out the first downshift and when it happens a 2nd time (and the engine light comes on), I can sometimes rev the engine to get it to shift into a higher gear, but I usually pull over and turn the car off. The check engine light (and the shifting problems) disappear when I turn the car on again.

The car is now almost undrivable (since I have to pull over so often), but if I can get above 45mph (into 4th gear) I don't have these problems. I've also found that coasting can alleviate some of the shifting problems. If I can coast from 4th gear to a stop or can take corners in neutral, there seem to be less shifting problems.

Does this sound like a transmission or a computer problem?