Hi, I have a manual/petrol toyota previa (S reg) MPV. Its done about 98K miles and now starts to overheat when on the motorway above 50mph -however when you put the heater on full blast the temperature drops suddenly and then rises when the heater is turned off. There is no problem when driving locally. Is it the water pump, oil pump, head gasket or the fan itself - as I think the fan is connect directly to the fan belt and has a heat coil that lets the fan stiffen 'turn faster' at the pace of the fan belt. Please advise.