If you have the DSG trans, it should be serviced Every 40,000 mile. I'm guessing it has never been done. At 120,000 miles it should be on its 3rd service as well as timing belt change, and possibly several other maintenance items. As far as trans problems go, you are the first to make the claim about the trans change and slippage. Also have an AC shop check your AC. It should work as good at 90 as it does at 80.