Thank you for the numerous suggestions. I think we'll try taking another
compass with us in flight for comparison, we still have the whiskey compass
that we replaced with the vertical card compass (and might be going back
to). Also isolating the compass better from vibration sounds like a good
thing to test.

I should have mentioned that the compass is mounted on the windshield in a
1966 Cessna 182 so it is well above the electronics. We did some of the
tests with the avionics master off and we switched off all strobes, lights;
it did NOT make a measurable difference. We're in the Sacramento Valley in
California and have repeated the tests on different flights so that we're
convinced it is not a local disturbance, nor have we seen the lost squadron
near Bermuda, but those are good points to bring up. And as I mentioned we
also accounted for wind on all tests.

Thanks to the group for the ideas, we're still listening if anyone has
suggestions.

--Michael A.

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Here is a summary of the suggestions we received so far ...

Maybe you can borrow another aircraft compass or even a handheld one (know
someone with scuba gear?) and set it beside it to see if it gives the same
errors - if it does, look for a localized disturbance.

Most airplanes tend to shake a lot more at high RPM on the ground than in
the air.
Since you say the compass does not swing smoothly in the air, try tapping it
with your finger and see if the thing is sticky.

Get a new flashlight? Mine had a big magnet in the bottom I didn't know
about. The flashlight now lives in my garage.

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.

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.
Yep, forget the GPS. Use this method instead:
Line up on a taxiway or runway with a known, published heading and carefully
set the HI. Go fly on gyro headings 360, 030, 060, etc. and carefully record
the magnetic compass reading. You might want to try it with and without
lights, pitot heat, any other major power consumers. Return to your known
taxiway or runway and verify the HI hasn't drifted.

You don't live near Bermuda, do you? I'm half serious....
Then again, I'm guessing that at one time your compass was working fine....