I used to own a 1969 Mooney M-20C Ranger. I know your's isn't a
Mooney, but I can tell you of similar problems and how I was able to
fix them. Perhaps you might benefit from my experience. Mooneys have
a steel cage with non-stressed aluminum over the cockpit forward, and
use monocoque aluminum construction aft of the cabin. The wet
compass, an Airpath, worked just fine for at least the first year that
I owned the airplane (1996-1997). After a time, it would exhibit up
to 40 degree errors, and this was an intermittent problem.
I researched the problem and found that one way to fix the
problem was to degauss the steel cage. Seems that there are a lot of
ground loops in the plane, as most systems just use the aluminum as
ground. The currents can eventually cause the steel cage to be
magnetized. Or so it is said. I got lots of advice on how to do it.
One of the better Mooney shops can rent out a de-gausser.
Alternately, I heard that they are available at a local TV repair shop
(although I doubt that many folks get TVs repaired any more). The
device, as I understand it, wraps around the steel tube. The operator
presses a button, holds it down, then removes the device from the
aircraft before letting go of the button. I heard that it was
sufficient to wrap it around the post in the center of the windshield.
Now for the confusing part. I had advice all over the map. One
shop said that the procedure described above was all I had to do.
Another said that I had to remove all electrical equipment from the
panel before doing the process. That is, take out all radios, and all
instruments. Even the ammeter and fuel gauges, etc. For anyone who
has worked on a Mooney panel, this is a very daunting task. Since I
couldn't get consensus, and didn't know if the procedure would even
work, I continued investigation.
Since the problem was intermittent, I asked the folks flying the
plane what the conditions were. Here in Colorado, we have a pretty
dry climate. It seemed that the problem was worse if there was
precipitation, even if just misty or foggy. That led me to suspect
P-static. I drove all around my home field and looked at the Mooneys
taht were tied down. None of them had static wicks, and neither did
mine. All the other brands of aircraft had at least a few.
I decided to spend about ~$300 on a static wick kit. This
included 15 wicks (three each for ailerons, elevators and rudder), and
the threaded attachments that rivet to the control surfaces. It also
included all of the STC paperwork. The wicks themselves were the
solid jet style and were about 3" long. After installing them, the
compass problem went away.
Just my experience. It may not apply to your situation, but you
may have a look at the static wicks on your plane to see if they are
in OK condition. I also have owned a Cessna single, a T207. It had
about 3-5 static wicks, the flexible rope style. I now have a 310
with a few of the rope style. I haven't seen any effects of P-static
on either of these planes, and have flown a lot more in instrument and
skuddy conditions.

Hope this is helpful,
Brian Cox