tach problems

I have a 03' yamaha 150 TXRB, with teleflex anolog gauges. Recently tach has been going haywire. At idol it shows 3000rpm and when running it may show 7000, even though I know by sound and speed it may be around 2500-3000. Then it may start working again, but only for a little while. Also fuel gauge shows empty at idle, but moves up w/rpm's. how can I check to see if its the gauge or sending unit on motor?

[boats.net]

Check for 12 volts at yellow wire on tach and fuel gauge with key on and check grounds. If thats ok then probably a bad tach.

Thanks. I checked voltage, actually getting 12.8 volts. Ordered a new tach so hopefully thats the problem. will let u know.