Thermostat?
 

I've got a 2004 MINI Cooper. Lately, the temperature gauge has gone from normal to overheating, though only while my car is idle. When I drove away from the light or whatever I was stopped for, the needle drops back down to normal. I took it to a mechanic who could not reproduce the problem, though he had hooked my car up to a computer and confirmed that it wasn't actually overheating. In the end, the car was returned to me with no repair made. The problem is still occurring, now more frequently, and I need to take it back to the shop. Can you tell me what could possibly cause this, so I can steer the mechanic in the right direction?

Are the electric fans coming on ???
Thermostat or radiator caps are also likely causes

yes! The fan comes on when the gauge goes high, and sometimes when I turn off my car.

I just had my car looked at after the thermostat was reading hot. It seems that the thermostat cap has cracked and is leaking fluid. there is no fluid on the ground when I park though.
My mechanic also told me that the replacement part is on SUPER back order in the Southeast USA. There is usually a date listed when the part will become available again, but there's no date listed and he said the dealership in Charleston has been buying new engines to get the parts they needed just so they could get the customers Minis off the lot!