Overheating question

My 92 SVX's temp. guage is going crazy in stop and go traffic. I can drive for an hour on the highway and the needle never goes above halfway. As soon as I get off the highway and hit a couple stop lights is starts to slowly climb towards the red. Once I get back up to about 40MPH for over 15 sec. or so the gauge goes back down (very quickly).

Anybody know what would be causing this?