MGTF 1.8 TRANSMISSION PROBLEM

[Barndweller]

Just picked up a 2001 MGTF triptonic 53k on clock , drives well when cold but as the engine gets hot it seems to randomly select its own gear which results in high revs and eventually i have to stop and wait for it to cool before it behaves , but prob comes back when hot, it happens in both normal and sport mode and started this year after its last mot in feb any ideas what this is and how i can put it right please all ideas welcome ..

[MaddAussie]

I suspect someone has put the wrong transmission fluid in the box It uses a very specific oil anything else causes problems