My temperature gauge says my car is over-heating when it's not, it goes straight up to the red and the light comes on within five minutes of being turned on. I've replaced the thermostat twice with no luck. i've noticed that there has not been a reduction of coolant from the reservoir. I'm wondering if a flush may clear this up or if it's water pump related. Any ideas?

 

Could be:
Air in the system.
Faulty temperature switch.
Bad water pump.