Transmission

2001 GMC Sierra 2500HD  6.0 Engine.  GM 4L 80E Transmission.  Problem:  Sometimes it shifts very hard.  Other times it is fine.  One day traveled over 200 miles with no problems.  Next time shifted hard on a 60 mile trip.  Only has 58,000 miles.  Transmission oil is clean and bright red.  I only drive the truck a few times every month and not at all during the cold winter months.  Mechanic found no codes when he plug in his computer under the dash.  Does anyone have any ideas?

Does it shift hard while shifting into every gear or only some? Heavy acceleration? Issue only occur when shifting up or also when downshifting?

Only shifts hard on take off and gear two and sometimes three.  I don't know about heavy acceleration because I accelerate slowly.  I am a slow driver.  Does not occur when downshifting but it has made a loud noise two times (but not consistently) when I placed the transmission in park.

It could a different things. Try disconnecting the battery overnight and see if it goes away for a while. It could be a sensor or could be something more. If it goes away its a dirty sensor but if not then it could be a pressure control solenoid or a valve body...etc