2002 chevy express van shifts gears hard after hwy speeds. on 2002 Chevrolet Express 1500

After driving hwy speeds the van sometimes shifts hard into gear. But it clears after the van is turned off for about 5 min and restarted.

Asked by for the 2002 Chevrolet Express 1500
how long has it been since trans was serviced with filter change?
I just brought the van about 3 months ago. At that time I was told that the trans was serviced @ about 150K.. It now has 166K on it. Again the hard shift stops once I turn the van off and restart it.. My mechanic looked at it and said the fluids were clean and didn't look like the trans needed to be rebuilt. Is there anything else that could be causing this problem.
yes alot of things solenoids and internal issues. did your mech ck for codes and look at live data to see what may be going wrong
no i dont think he did I will ask him. Thank you for your input and your time..
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