DD,
I had the same problem, wandering SW tach.
It would be ok for a while then take off on its own, stay in one place...
The best price I found on the internet 3 or 4 years ago was through XK's Unlimited. I think it was about $90+...
http://www.xks.com/
The new tach has DIP switches to set the number cylinder, the old tach had a rotary switch to set "4-6-8".
The down side on the new tach is the face of the looks like the rpm increments were drawn by hand...but, it works.
Pete