erratic water temp gauge
 

I have a 1999 Freightliner classic with a series 60 430 hp detoit. My problem is I have an erratic water temp it will work fine then for no apparant reason it will fall to the bottom and inch its way back up to 150 degrees then fall and again inch up . this will happen for days then clear up . Ihave replaced the guage and temp sending unit ,also when this occurs the engine fan kicks on any help would be appreciated