Did you check thermostat before instalation? Have had some crappy ones lately. Check in boiling water, had one I checked a few times while out of car and would take different times to open and would intermitly open at different gaps sometimes barley open sometimes open all the way, sometimes took a long time to open. See thread titled is this head gasket? in engine issues section. Just a thought and alot cheaper and easier than head gasket. Good Luck.
