My 2001 shifts hard and the check engine light comes on. I havent had the CEL code checked yet.

It drives fine except after driving for an hour or more then it shifts hard from first to second. No problems in other gear ranges. The next day after it sits and probably cause it cooled it drives fine. The CEL goes off after a few days.

Anyone have anything like this?

Any ideas?

There is a line pressure modulating solenoid valve in the transmission control valve body. This valve cycles open and shut rapidly to control pressure. The solenoid operates a valve spool which can wear and stick, causing erratic line pressure. Low pressure, and the shifts can be sluggish. High pressure, and the shifts can be harsh. There are many other possibilities, but a fully capable scanner can test the solenoid and even modulate line pressure via external input. You might not even have to remove the transmission pan or raise the vehicle to diagnose it.

its your manufacturers transmission control. there are 2 sensors underneath the tranny where all the fluid is kept. each sensor is bout 100 bucks, we werent sure exactly what one it was, so i replaced both of them. works fine now. BlazerLT had responded to my post bout this issue before and also informed me about a transmission ground update. something like 57 bucks at your local dealership. it think that was bout it, might wanna loook into that to. mine is a 97, so your sensor location might be a little different, haynes manual should tell you.