Could be several things, including (but not limited to) an air bubble (I have experienced that), bad sensor (or wiring to it), or the fan system. I have a description of how and when the fans should work
HERE.
What you really need is a GOOD mechanic (or friend) that will take the time to actually troubleshoot the problem instead of just "taking a shot" at replacing parts that MAY fix the problem.
I suggest that you ask EVERYONE you know for recommendations about such a person. Once you find them, stick with them.
Take Care,