2000 K3500 Losing Overdrive

[sagent]

A buddy of mine picked up a 2000 1 ton old body style Chevy with a 5.7 L and the transmission has been acting up. After about

5-10 minutes of driving at 55 transmission will suddenly downshift to 3rd and not go back into overdrive. At the same time

transmission with shift hard into 1,2,3. After stopping & shutting down for a few minutes, truck seems to shift smooth and overdrive works.

 

Fluid is right at the mark and appears ok colorwise. No codes stored in the computer. It does not do it every time .

 

Any ideas where to start with this problem or anyone else with these symptoms?