Thermostat question

[jalal@lecap.engr.sgi.com (Jalal Rajabali)]

I finally got a new temperature sender unit installed on my 1988 Audi 90 Quattro and the temperature guage started functioning again.  But now I notice that
the temperature gauge doesn't stay constant.  
When idling the temperature gauge will rise but once the car is cruising
on the highway the guage will creep down to the "C" (cold) reading.  
COuld it be a busted thermostat?  If yes, then how hard or easy is it to
change one?  BTW, the car has 124k miles on it.

Thanks,
Jalal