99 silverado 1500 shifts hard after its been driven a while

Hi, my 1999 chevy silverado 1500 6 cylinder extended cab shifts really hard from 1st to 2nd gear after its been driven a while, whether its hot cold outside makes no difference. It seems like after its been driven for @ 20 minutes it starts acting up. If I pull over and shut the truck off and turn it back on the problem goes away for a bit but will usually return after 10 minutes or so. It will run me 115.00 smackaroos just for an estimate. Any help would be very much appreciated.