I know you said that you replaced the radiator cap, but I recall reading that the Mishimoto ones are crap. If you can get an oem one, that may solve your issue. On a side note, is your upper turbo coolant tank full? Is your entire system full of coolant? If you aren't overheating, it either sounds like a pressure buildup (perhaps not enought coolant) or the radiator cap is weak. Good luck.