Hard Shifting after 30min of highway driving

My 99 4x4 Blazer LT have a problem shifting hard between 1 and 2:nd gear
after 30 min driving on the highway in 70-80mph.

And when the truck is starting shifting hard I only have to stop and turn the vehicle off
for 10-15 sek and start up again and then are the shifting normal again??

Anyone with the same problem??? ....and maybe a solution on the problem!

Best Regards
/S

all me and my bro could think of is computer or something in the tranny, because its electronically controlled tranny.