1996 Passat GLX transmission

I have a 1996 Passat GLX VR6 with AT. The car has 37K original km. The transmission is noticibly slipping between 2nd, 3rd and OD. At highway speeds the transmission can be slipped with moderate acceleration. I have stopped driving the vehicle until I can understand the problem. Cannot check the oil level as there is no stick. I have loosened the oil drain plug and released a small amount off oil to determine if there are any metal filings - none detected but a definite burnt smell to the oil.

Any suggestions?

You can try changing the fluid & filter, but it doesn't sound good.