Hello, I am the 2nd owner of a 2000 Regal LSE, 112,000 miles, Series II 3800 motor with automatic trans. As far as I know its always been adult driven I purchased it with 84k miles on it. The last 30K miles have been highway miles. I have just recently noticed the transmission beginning to shift hard after the car has been driven for about an hour and completely warmed up. Short trips car shifts fine. What doesn't make sense is sometimes I'll drive for an hour - let the car sit for 30-45 minutes, afterwards the tranny shifts fine. Transmission oil level is full. Which makes me wonder if this isn't more of an electrical issue? Is there an electric shift solenoid located somewhere? If so, is this something a avg person can change or will I need to take it in to the shop? Any suggestions and directions to fix this issue would be appreciated. Thanks for your time.

Possible pressure control solenoid - PCS which is located under the transaxle side cover assembly. Get it scanned and see if a P1811 DTC is stored (max adapt, long shift).

Replacing the PCS is not a typical DIY job.