Might have to move the compass up away from the panel. There are a lot
of electrics under the glareshield that could be interfering with it.
I had the same problem with a vertical card compass in an Ercoupe, and
had to put it way up on the windshield bow.
Those vertical card units are very sensitive to vibration. You HAVE
to use the really soft, floppy mount they provide for them.
Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading
indications for compass calibration purposes. Use the heading
indicator instead.

Dan