Rpm unstable

Hello everyone.When the vehicle speed is about 100km\h the rpm goes up&down around 50rpm+-.
what can cause this Phenomenon?
thx
discovery 1997 edc

Actually the rpm ups&downs are more of 100-200 Difference...

The pick up at the back of the alternator is dirty or lose had the same problem cleaned it put it back on problem solved

thx but its a 300tdi...

as Gazdav says,

pull the grey wire off the back of the alternator, give it a clean and the spade connector on the back of the alti, put it back on,

and check, did this on mine, OK now.

Cheers

No probs glad I could help

i thank you all for the Solutions.but its not only the Gauge, the rpm are up&down and i can Hear&feel it.

Ah so the plot thickins ok seeing how it is edc have you removed the EGR valve and when was the last service.

the egr is out and the truck is well serviced.last one was a month ago

Ok then try the following

Check air filter and air inlet is clear and if the inlet manifold was cleaned when EGR was removed
When serviced was valve clearances done
Dirty injectors clean with injector cleaner or remove and soak in diesel over night
Is just a few causes of that problem I can think of

ok, thx.i'll let you know Findings

btw can it be the fuel pump?

Yip but unlikely due to the very small movement in rpm if any thing it would be dirty fuel lines blocking the pump.