my outside temp indicator behaves just like this. i have noted that this happens when the ambient temps are around 70+. in the cooler temps recently it was rock solid. a few have reported success with just replacing the sensor:
http://www.peachparts.com/shopforum/showthread.php?threadid=7854&perpage=15&pagenumber=1
in any case it is the least of your worries as gilly indicates. those two major repairs he mentioned easily add up to about $2k (BTDT).