My heat does not work. I notice the temperature gauge thats on the dashboard fluctuate back and forth nearing the H. The mechanic did not detect a leak, and suspects the thermostat is ok. He said the hoses appeared to be intact. Now he thinks its air pockets in the system. I've never heard of that. Could air pockets cause my heat not to work? Is this common? I've had my car since 2005 and this is the 1st.