When I enter onto an interstate (mainly go above 50 mph(, my car seems to be stuck in 3rd gear and won't go up to 4th. So I end up at 5000 rpm or more if I try to maintain speed.

This doesn't happen every time I go over 50 mph. There is no pattern - hot/cold, been sitting / just driven, etc.

I have to pull over or take an exit, turn my car off, turn it back on, and then I can speed up and it will shift like normal.