Subaru Repair: My Subaru coolant system, coolant system, head gaskets

Question
My dashboard gives me readings of engine overheat after every 5 minutes drive. Now I checked for leakages,(none) even flushed the whole coolant system and replaced the thermostat. But the problem persist. The fan works normally but one side of the radiator gets very hot while the other(inlet of coolant) remains cold or normal. I am considering a that the leakage is internal but still no signs of any leakages any where.
Can you suggest something I could check again,coz I am out of options.  

Answer
Head gaskets .Can be difficult to Diagnose.Start with simple stuff after it over heats smell the overflow bottle and see what it smells like.Chances are it will have an odor of exhaust,fuel or sulfur none which belong in your cooling system.

Justin
All Wheel Drive Auto