GF's EK h/b overheats from time to time. After letting the car sit and cool off I check the water levels and its kinda low.

 

bad headgasket, leaky radiator/hoses, heater core.

 

most likely you have a blown head gasket. you should pressurize your cooling system for leaks first. There is a possability that your thermostat is stuck shut and when your car overheats you loose some of your coolant out of the overflow tank.

 

No leaks?

You should have a leak-down test done to it. Some times it's an internal leak, like a head gasket, etc...

 

i use to loosa alot of water around my thermostat