Cadillac Eldorado can’t choose between gear 3-4

Hello all, I have a 95 Cadillac Eldorado. Recently he started to jump back and forth between gears 3 and 4 all the time, as if he didn’t know which gear he should turn into. If I switch manually to the third gear, the problem is gone. The problem is not always there right away, sometimes it only comes after 10-15 km, so if the car really comes to temperature. Can someone help me there? Greeting from Cologne