I have a 2002 impreza outback with 190K on the clock. I was losing coolant and the top seam on my radiator was wet. I took the radiator cap off and ran motor until warm. The level on the radiator rose until coolant spilled out from where the cap was. I looked at the level and noticed every 10-20 seconds a air bubble arose. My head gaskets waterpump and thermostat was replaced at 135k so It cant be head gasket again???? what else would make bubbles in my cooling system??? thanks a lot!