Cold engine or faulty temp gauge?

[Mike <mac50@columbia.edu>]

Just got back from a long trip in the ol' 89 100 and was disturbed midway
in the 5 hr trip when I noticed that my water temp gauge apparently
stopped working.  It read below the lowest mark on the "cold" side.  Oil
temp remained very close to 80C throughout (as it always does).  The trip
ended and I parked the car and turned it off.  Then, before getting out of
the car we noticed it was a no parking zone, so I fired her up to move
and--presto!--the gauge now registers about 1/4 to 2/3 of the max.  

So:

1.  Anyone ever have thisproblem? 
2.  What's it cost to fix?

And BTW, thanks to everyone for your previous answers!

Mike