got bmw 316 touring, 1997 e36 model, had head gasket done last year cos it was building up too much pressure in cooling system, had gas crossing over into coolant, now i have similar pressure problem, AA say it may be head gasket, did sniff test but result was inconclusive, my mechanic thought it was water pump but it wasnt, could it be the thermostat not opening, any ideas would be appreciated.