[Ben]

It's not very common, but this does happen with factory Mitsubishis. The cap will let coolant out when it reaches it pressure rating, but then won't let it suck back down when it's cooling down after the engine is off. Just like when you said, leaking coolant while driving, radiator is low, and overflow is full.

Another thing someone said about not running a tstat is true. In the winter it won't even warm up completly, but in the summer it will overheat cause the coolant doesn't stay in the rad long enough to get cool. You can get washer looking things that go in place of the tstat that have different size openings. Just have to play around and find the right size.