Tachometer on '06 Mercury 60hp 4 stroke

[Oldchief]

Tach on my 2006 Mercury 60hp 4 stroke has been showing false RMP readings and bounces all over the place. Gotten worse lately. At fast idle of maybe 1500 rmp's, it will show anywhere 3000 to 5000 rpm's. Is this caused by a bad sensor or what? Thanks!