Hello, have a problem with my automatic transmission:

when cold, everything works fine. After a while, it will start to not shift into the next gear, rpm goes up but doesn't shift. kick down will rev the engine, but doesn't downshift smoothly.
it can be driven with a "sensitive gas foot", and drives quite normal then. it doesn't shift hard or anything, the problem is just that it won't shift up correct, or down with kick down ... when the engine is warm.
Any idea what could cause the problem?
The car has run 95.K miles.
And please excuse my English, I´m German, which makes it even more difficult, these cars are rare over here.
I have never seen any other 2.3 Ranger over here then mine.