'90 80q gauge issue

[jason nyby]

My fuel and temp gauges fluctuate quite a bit.  They also do it in harmony.
I am thinking that maybe they share a ground wire or something.  Sometimes,
when I fill the fuel tank, the fuel gauge will only go to 3/4 full and stay
there til the tank is 3/4 full and then go down as the tank empties.  The
temp gauge is my main issue.  It really bugs me to not know how hot the car
is... Any ideas?

Jason