2003 Dodge Durango severe temperature swings

I have a 2003 Durango - over the last several months as I drive, the temperature gauge will hit the hot mark then back down into the normal range. This is most pronounced during in town driving where the speed rarely exceeds 40 and there is more stop and go. On the highway it rarely happens. I replaced the fan clutch this morning, thinking it was the problem - but it's still doing it. It gets hot, then cools down like a switch is being thrown - just too late."