If it is (high frequency) transcients coming in on the +12VDC/Ground
lines from the engine that is causing your vario problem, then one
suggestion is to retrofit a torroid on the +12VDC/ground lines.

Like these. http://parts.digikey.com/1/parts/185...a2005-0a2.html.

Might help. Couldn't hurt. Good luck.

- John