Battle Wagon baffles mechanics

A couple mechanics have had their head under the hood of my Subaru Station Wagon over the last couple of days. They have no clue what's the cause of the seemingly random overheating problem. It has a new water pump, thermostat, heater core, radiator and cap. Right when we think all the air is bled out, it gets hot again. It seems like something isn't working right, but what? Can it be the mouth on the radiator is bent? Can it be a minor head-gasket leak?

Today the mechanic actually complimented me on how well that car runs, saying that it has lots of power for an old car with high miles. What's even more amazing is that everything works in that car, still.

Where do I go with this next? I'll keep you all posted.