rpm gauge needle jumping

[beerman]

1983 300sd turbo Mercedes with over 245,000 miles on it with a rebuilt engine at 198,000 or so miles and I am having trouble with the rpm gauge.
My rpm gauge needle suddenly jumps back and forth as high as 55rpm and back to zero. It does this a few times then rests at zero. sometimes it comes back to life in the course of an hour drive and other times it does not.
When I start the car after resting an hour or so the needle seems fine, but after a time the needle jumps again and rests at zero or returns to normal.
Does it have anything to do with the cruise control, because I am using that when it happens some of the time, other times not.
Any ideas????
Thanks from a new poster
and a new Mercedes owner!!!