Crazy Gas Gauge

[tenkiller@domain.elided (ten killer)]

I have a '91 Alfa Spider.  For about 8 months now,  my car has been
having a peculiar problem.   The Alfa service mechanics have tried to
find the problem,  but can not.  The problem is this:  The gas gauge
registers normally until the headlights are turned on.  When they are
turned on,  the gas gauge registers past full,  no matter how little gas
is in the tank.  But,  not all of the time.  It only does this when it
is cool,  damp,  or night.  If I turn the lights on in daylight,  the
gauge does not change.  When it turns night,  it registers crazy.  But
not all of the time.  If it is warm at night,  it registers normally.
Occasionally,  it will be crazy at night but after a while,  it will
right itself.  If it is raining,  it usually does not register
correctly.  Also,  when it is acting up,  I can dim the panel lights and
the gauge hand will follow the reostat until it hits the true gas level.
When I turn up the lights again it returns to past the full mark.  BTW,
the particular light for the gauge is dimmer than the others when the
gauge is crazy.  It will brighten back to normal when the guage corrects
itself.  

When I have had someone look for the problem,  it works correctly.
therefore,  they can not find anything wrong.  The gauge has been tested
and does not need replacing.  The sending unit seems to be ok.

Any ideas?
thanks
tenkiller

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