Bouncing tach, what to look at first

[MrsRobinson]

Port engine tach goes crazy over 2000 RPMs. Seems pretty true at idle and under 2000. Once on plane the starboard will show 3000 while the port will show 4000.

What should I look at first? New tach, new cable, something else? Any simple tests I can do to start ruling things out?

Thanks,