peugeot 206 2003 automatic - gear box slipping?

When the engine temp reaches 90 degrees, the gears slip or miss altogether and I can't move for up to 15 mins. Then it finds a gear and I can go again. This happens from stationary, slow speeds not sure about moving? When temp is below 90 it doesn't seem to happen.
I have just had gear box oil changed and it is still happening. Can anyone help?