If the vehicle is a manual transmission then is sounds like the clutch is on the way out. If its an automatic gearbox firstly you could check to see if the transmission fluid level is ok.
Either way I suggest you have the vehicles transmission inspected as there is likely a clutch issue that requires attention.
I've had these symptoms on my car as well. In my case the rubber parts of the "support arm(?)" was worn. I don't know if the translation of the word I'm using is correct, but it should look like (any of) this:
The Audi dealer said the repair would cost ~600 euro for replacing all four.
I bought the parts (4 pieces total) for roughly 15 euro each and ...