Gearbox slips through after a lot of city traffic

Hello, If I travel in the city for a day at >30 degrees and then drive out on the country road, the transmission slips through. I can’t give more than 60-70% gas. Otherwise, it seems like it jumps into the neutral passage. I have to go off the gas for a short time to catch it. After that I can carefully further accelerate. That’s always after a lot of city traffic on hot days. Oil is just 15k km old and still nice red. Can I do something? Thanks Lexenlewax