I have a 2000 regal GS. Trans shifts good when cold but after it warms up there is a shutter and seems to take slightly longer to shift.

Welcome to AF.

Best thing to do is get it scanned with a trans capable scanner and see if a DTC of P1811 is stored. This code does not trigger the SES/CEL light. Common problem with 4T65E autotransaxles is the PCS solenoid. When was the last tranny service? Pan drop with ATF and screen filter. Also, what is the current mileage?

I am unsure of the last service. I just recently bought the car. Mileage is 140,000. Where is the PCS located, is it excessible from dropping the pan? Thank you

With that mileage and since last service is unknown I would start with replacing the ATF and the screen filter.

The PCS is located under the tranny side cover and not exactly a DIY job or minor repair job. The engine and tranny must be dropped with the cradle/subframe to access it. See pic below.

http://i5.photobucket.com/albums/y186/lizzywiz/valvebody2.jpg
EPC Solenoid = PCS Solenoid