I have twin gaffrig tachs and one of them occasionally will jump around (the needle). What would cause this and where should I start to look for the problem?

Some of the older tachs get interference from other electrical sources that may be too close.... I know that Gaffrig has a small circuit board that you can get from them that gets bolted to the back of the tach for this....Give them a call....i may even have one or two sitting around here someplace...i'll check later tonight.

Hope this will help, I have been told of a problem with grounding can cause this. I have a '99 Formula with an HP500EFI, supposedly they painted the block and then put the distributor on. This caused a grounding problem at teh distributor that only shows up on the tach (boat runs fine), doing exactly what you indicated, jumping around (200-500rpm) occassionaly.

I found one filter if ya want it....

Bouncing tachs are usually due to secondary ignition problems. Moisture, bad cap/rotor, bad plugs or wires. You will occasionally see a bouncing tach before noticable engine problems.
Do the simple stuff first, ie plugs and wires.
Gary