2001 Buick Century Rough shifting after driving awhile

After driving around town for 10-30 minutes, my smoothly shifting Buick switches to shifting from first/second and second/third jerkily. I also can hear a whiney noise which is not present when I first start the car.

My mechanic changed the transmission fluid last week and cleaned out a filter. It was fine for a couple of days and then started in again.

I first noticed it driving into work, which takes me about 15 minutes. It started it at the ten minute point for no apparent reason. I drove around town today running errands and it did not start it for quite a bit longer. I have seen various suggestions on the internet that include a pressure regulator solenoid, a sticking regulator valve, a bad pressure switch, a computer problem, or wear of internal transmission parts.

I do not want to replace the full transmission as the car is 9 years old and it isn't worth it, but I would like to get a few more months out of the car while I pay off a few other bills. My mechanic (who is very trustworthy) is calling a transmission friend of his, but I would like to see what anyone here thinks. Thanks so much!