Why does my transmission gears go out when hot?

Check the trans fluid first. That's the first thing to always do. Could be low that will cause gears to fail. That go from there. It might say how to check it on the stick. Some need to be warm and in park to check. Some in neutral.

Suspect pressure control solenoid inside trans pan.. Not a DIY fix!.. Can be scan tested with proper scanner, forget about the parts store - *parts salesperson* with some kind of code reader!