Rear engine tach is not right
 

I have taken a few flights lately and notice that both the rear engine tach and primary cht gauge are bouncing around. Looking at the attached video you will see that the manifold pressure is constant as is the fuel flow. No issue from a performance point of view. The prop sync does not seem to want to work either. Has anyone seen this? Ideas?

https://photos.app.goo.gl/Zwa22jZUmFTSw1gT7

Check the rear engine ground strap

So. I found the problem today. There is a coaxial wire that runs to the right rear mag, actually goes to each of the mags. However the ground wire was broken off from the crimp connector. I replaced and now tach is solid. My guess is it was also hurt overall performance of the engine.

When checking out the wires there was also a ground strap to the rear alternator that needs to be replaced. I had no primary wire but will do this soon as well.