Tachometer erratic readings

[Preferred]

I have a 2010 regal 2300 with Volvo penta 5.7 Gi-J engine. The tach is giving erratic readings. From showing 6000+ RPM to jumping around from 2000 to 6000 RPm. I have checked the connection at the rear of the tach and all are tight. Not sure where to go next to check.

 

[alldodge]

Use a hand held Tach and see if it does same thing. Can get a loaner from auto parts place in most cases.