Oldsmobile/Buick Repair: 2002 buicki century, intake air temp, coolant temp

Question
Hello, I have a buick century custom with 3.1l engine, it was leaking coolant, we had the head gasket changed as you could see it leaking there.  not leaking now but when driving especially stop and go traffic or after about an hour the temp gage goes to the halfway mark then the transmission starts shifting roughly, any other time the transmission is very smooth, since halfway on the temp gage is not actually considered overheating what could be causing this, I've seen posts on a couple other sites on this same issue, but no answer.  it also did it before the gasket change  I appreciate any advice.

Answer
Hi
Is the service engine soon light on?
I would consider going and having the PCM scanned for codes EVEN if it is not on.
I would check the actual coolant temp with a scan tool also, to make sure it is in sync with the gauge.
I would also check the IAT (Intake Air Temp) sensor, and make sure it matches fairly close with outside temp underhood.
First I would check for codes
If some are retreieved. please follow post back.
Pawl