2002 Pontiac Grand Prix My temperature gauge plays around

Hey I have a pontiac grand prix 2002 automatic and it had problems at first with over heating and it did over heat. But all leaks in the vehicle have been fixed and even a thermostad was replaced a coulple of times, also a solution was put in to save me from an expensive head job but, the car runs good the teperature gauge go's up and down weirdly like the car is going to run hot the fans cant be the problem because they come on when there suppose to my mechanic is even having a difficult time solving this problem can anyone help me out?