There seems to be 2 things that come to mind. If you open the radiator cap, what color is the coolant? What color has the coolant always been? Red(dexcool) or green?
And which ever color it was, did the opposite color ever get poured into the radiator?
If so, then coagulation may very well have taken place, and the innards of your coolant system may be gummed up. See the links below.
look at pics
The other option is that it may be a bad head gasket, or small fissure in the block/head.
1) To check this you can let your car sit until the engine is completely cold.
2) Open the radiator cap and make sure the coolant/water level is toped off in the radiator.
3) Making sure the motor and coolant are completely cold, start the motor.
3) Then immediately open the radiator cap.
4) If the water/coolant gushes/bubbles out (not dribbles over the stem/neck that the radiator cap secures to) at idle or if you rev the engine(via throttle body is easiest) then there is a good chance the head gasket is bad or the block/head has a crack in it.
If it is any of these I am hoping for the first and not the latter.
Hope this helps