Might start with simply checking the tranny fluid level. Sounds like it might be the better part of a quart low. Continue driving with the fluid low and you'll be looking at a tranny rebuild.
If it is low, then top it up long enough to get a new filter and fluid. Be sure to check the level with the engine/tranny hot (80*C) for proper fluid expansion. You can get a fair read cold - a full tranny, when cold will have the fluid level about 10mm below the bottom mark on the dipstick.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.