I have a 2000 Silverado 4X4 with the 5.3L. On warmer days or while pulling a trailer, the transmission shifts very hard from 1-2 and 2-3. I have 88k on the fluid. Only seems to do it when the outside temp is 70 or higher and after I have been driving for 30+ miles. Seems to be related to heat but there is no smell or noise and the fluid looks good. Any suggestions?