Temp gauge issue
 

My temp gauge in my 2001 xj is acting wired. I start the engine and temp gauge gets to de midle during 10 mins aprox , then nedle starta to go down slightly to 0, and staya there. If i tuen off the engine and atart it again, needle goes up to the midle to stay there during 20 secs and starts to go go down again and pattern goes on and onany clues to fix it?
thnx for your help