Hi Thanks for your question my name is spanky.
First check air pressure to trans just to make sure you don't have low air pressure as some trucks gage will show good air supply
but they have check valves inside air tanks that go bad,, if the trans is on bad side it will cause problems like this.
Next This sounds like it is a low voltage issue,,what happens as things heat up you don't have enough amps to complete the shift.
Next to trans is a 6-8 way plug that has all your power and grounds for trans,,you want to check that harness all the way back to
You need to clean all battery positive/negative connections including frame, starter, and engine.
Most times this is the issue.
If you run into trouble let me know I'm happy to assist.
FYI I will be out for a few hours if you end up needing anything else follow ups are included after rating so no worries.