Before I spend money I figured I would get advice on this problem.  My tachometer needle is jumping around and showing the incorrect engine RPM on my '89 GS500.  I checked the cable and it is tight at the gauge and the engine.  There is also a loud "whirring" noise coming from the tach when it is acting erratic.  If I shut the engine and restart, it acts normally for a while and then the problem returns.  It seems to have started after riding down a bumpy road.  It also seems to occur more frequently on colder days.

Any ideas?

When the tach is erratic does it bounce above and below true engine speed, or does it always read true engine speed or lower?

Above and below.  Sometimes it stabilizes but at a higher RPM than actual.

Sounds like a binding or broken cable to me. Remove it and take the center out. If it's whole and not broken, you probably just need to lube it.

Thanks everyone for the replies.  Took the cable apart and it felt like it was "crunching" when I spun it by hand.  Sprayed some lube in there and it works perfectly now.

Problem solved!