Are your cooling fans working properly? Head gasket issues usually show contamination of the overflow bottle, brown coolant. The oil can be contaminated, getting brown with a milky white foam collecting on the filler cap. Any white smoke coming out of the tailpipe? When was the last time the cooling system had some cleaner run thru and flushed? How about a new oem thermostat? If your heater is blowing hot air I figure that the system is working with no trapped air. Steven.