Ok guys heres a poke at it. Have you, or had someone use a block tester on it? If not, do so, you may have a blown head gasket that is causing this. I have seen it with other makes. I know you stated you did not have the other symtoms like bubbles, but it does not matter. Still might be a blown head gasket. You may also presure test with a pump that screws onto your radiator where the cap fits. Pump it up to your cap pressure, watch the pressure for a drop, then as you run the motor see what goes on. The pressure should never go above the cap pressure rating. There is also an adapter to check your cap.

I hope this helps! Good luck.