strange engine temp. problem... need help

[BMW_325_i]

My temperature gauge is not stable at all. Eveytime i drive the car a little bit , then park it, it overheats. The weird thig is that it goes to the 3/4 line then it comes back without the help of the electric fan. I checked the temperature on the engine and is seems stable(always same temperature) even when the gauge is at 3/4 line. Is this problem a sensor somewhere or the gauge itself???? Any help would be very appreciated.

Thanksss

[Ski]

Thermostat needs replacing..

[Fox]

what do you mean you checked the temperature of the engine? Did you actually go put a thermometer to the engine and check or what? How often does the temperature sporadically go up and then come down and how fast?

[BMW_325_i]

I measure the temp with a laser gun. Its very accurate. You point it at the an objecet and tells ya the temperature. Also, I can tell if the engine is hot or not even without a thermometer. The problem is that the engine temperature is not changing even though the gauge tells me its overheating. Thats what i dont understand.