Andrew;

It MAY just have an age/dirt/corrosion caused intermittent in an internal calibration pot along the lines of the 1800 tach. (ref 1. *), which is fairly straightforward to repair. If it pegs, that means the meter movement itself is still OK, but but if the thing gets warm, this is not a good sign, I suggest you disconnect power to it before it fries totally, and have a qualified electronic specialist check it out. Once the meter movement is burned out, it's pretty much toast (NPI), and I don't expect there are too many replacements out there. I don't have internal circuit info, ask around for that...a post here would probably be a good start...it will make troubleshooting/service a whole lot simpler. If you want me to look at it, contact me off-line...and...

...damn, sounds like your tach is brocken, man...sorry, you asked for that...

* http://www.intelab.com/swem/service%20notes.htm#Gauges

Cheers