Tach Problems 12.7 DD60

I have a 1993 357 Pete with 12.7 DD60, I have been slowly losing the tach function. It started acting up with the needle bouncing around. Later at startup it would not move off zero until rpms reached 1000. Yesterday after warm up it worked fine for a couple hours, then bounced around again. As the day got longer it took more rpms to move needle off zero. By end of day had to mash pedal to floor to get needle to jump then it would fall back to zero. The preset pto setting still works as always, hit the cruise goes to about 1000rpms to operate loader off pto. The highway cruise works as always, no change. Pulled plug off back of tach, put a multimeter on leads. Did not see any voltage change with rpms, revved it up no change. Any ideas? Thanks