At first it almost sounded like you have a headgasket problem, but on the other hand it Sounds like you have air in the system or a somewhat clogged heater core. I would try to bleed the cooling system first. I like to get the RF corner of the car jacked up to have gravity help me a little bit. Top the expansion tank off and bleed from the t-stat housing and the overflow hose going to the expansion tank. If that doesn't work, you mite want to try a CLR soak and flush of the heater core. I used to do that to Audi's/VW's that came into our shop. They have a issues with heater cores clogging because the coolant never gets flushed out on a regular basis.It worked about 80% of the time. Give that a try before you investigate headgasket issues