940 Tach Jumping

Hello, anybody have an idea why my 940 tachometer jumps around. It jumps form 0 to 1,000 at idle and while driving it jumps around another 1,000 rpm. The car has 32,000 original miles (old lady driven) and the tranny is fine. Any ideas?

My first guess would be a wire shorting out. I don't know the wiring for the 940, but maybe be able to trace it.