Temp gauge go up and down

Ok my temp gauge run normal when it cool but
after it run at normal temp  the  temp  gauge would
up to half way mark and stay the there for like  a
min  and then it would drop down to normal
running and then it does it again  I took it to a
dealer ship to hav it fix  they didn't know  why it
was going it so they change out the fan clutch it
run normal for like 24 hrs now it is doing it again  
so now maybe some one can tell me what causing
it to do it so I can get it fix  and it is on  a 2006
dogde Durango with 5.7 hemi