It doesn't sound like a HG by the description of the fault. If you are getting exhaust gas in the coolant, the coolant will be blown out of the pressure cap. If there is no evidence of this, a HGF isn't likely.
If you don't trust the head gaskets, changing them isn't an impossible DIY job. The parts are cheap enough too.
But like I said, it doesn't sound like HGF to me.
