Got a problem with MK1 GS300. Coolant levels changing in expansion chamber and stream of 'bubbles` in the chamber from startup. This bubbling gets worse after few miles and is audible after parking up. Expansion chamber has just overflowed last night after short trip. Water not boiling, not steam as measured temperature. Engine temperature sometimes rises when idling. New radiator fitted recently and system pressure tested no problems. Above symptoms only started after service by non Lexus garage. Spark plugs, oil, air filter, starter motor changed. Coolant was low when I collected car. Mechanic said that a water pipe had to be disconnected to gain easy access to starter motor???!!! Are we talking head gasket here with gas entering coolant system? Or air lock? Any help would be appreciated.