Hi - I have an A4 TDI (1998) . When cruising above about 50mph, without changing the accelerator position, the revs increase by about 100 for about 2 seconds, then drop back to normal. This repeats itself every 5-10 seconds.

It is hardly noticeable to a passenger, but very noticeable to the driver. The impression is that you are going over a rough patch in the road.

Any ideas please? - David

Forgot to say its a 1898cc TDI