Hi peeps got a slight problem with my M reg Discovery the rev counter is a bit erratic engine is running fine but rev counter fluctuating by about 500 rpm any ideas

Could be a couple of things, could be the little wire on the back of the alternator that controls the rev counter is goosed, bad contact (take it off scrape it up a bit and refit) or is melted the back up from the turbo if the heat sheild is missing/non existant.

Or your serpetine belt is slipping slightly, check the tension of this, are there any squeals?

Our 300tdi does it as well, I just left it.