hi, had same prob with one of my rs turbo's, changed head gasket & thermostat, turned out to be a blocked heater matrix. hoses bubbling up & bursting,
solved prob by removing hoses going to matrix & blowing air pressure jet through matrix.
be careful to hold open end of hose away from your face, or you will get a face full of gunk or in your eye. then flush through with a hose. hope this helps.
pat.