I had this exact same issue, however towards the end when I finally figured out the problem, it turned out to be a bad radiator, and the driver side cap was leaking. I could never figure out where it was going, because the inside bottom of my front bumper was letting it puddle up against the rubber lining, then it would dump out once I got on the highway. Fortunately I caught it before it ever started to overheat, unfortunately for the replacement and labor it ran me almost $500 at a local shop.