93 K3500 454 4L80E Tranny problem

When the Trans warms up, it will no longer shift into 3rd or 4th. Also no compression braking. If cruising at speed, it will "freewheel" (engine returns to idle, no power until RPM reaches level for driving in second gear). Fluid is clean, as is filter. 50000 miles. Serviced regularly at dealership since new. It only does this if the ambient temp is above about 65-70 outside. When it's cold out, it shifts normally. I know it's heat-related, but what is malfunctionong?

update:Took it to dealership. They couldn't reproduce the problem. Changed the VSS due to a code 24. After picking it up, I started home and it did it again. Dealership said bring it back.