95 Riviera Transmission problem

I've got a 95 Riv with about 200K miles on it, just replaced the engine. I don't have a code reader, and don't really have access to one, so I'm kind of shooting in the dark here. Symptoms:

Scenerio #1: Start the car, no SES light. Run it easy, no SES.
Car runs OK, shifts in and out of overdrive.
Shifts in and out of overdrive suddenly become real soft
Then, car will drop out of overdrive and not shift back in. SES light comes on.

Scenerio #2: Start the car, no SES. Run it easy, no SES
Again, car runs OK in and out of overdrive.
Now punch the gas, and it downshifts fine, but SES light comes on. Transmission still shifts fine, though

So what the heck is going on with this thing? Any ideas on where to start looking first?

Thanks, folks....