2000 Pontiac Montana Tried everything, what now?

I changed intake gasket last July. Everything worked fine until about Oct. Occasionally it would overheat, and the heater blow cold. Always if the engine would even start to get over normal temp, the heat will blow cold. I have changed the water pump, serpentine belt, thermostat 2x, radiator cap. For a while I could bleed the air from the water pump valve and have no problems for that trip, but now that doesn't work anymore. Now the heat is always cold, and if I use low gear to keep the rpms up the temp stays down. Usually at first idle the temp takes a while to go up then stays about half. If I drive then the temp shoots up fast to the red and somtimes drops but not always. I thought it might be the gauge so I drove with it hot, and it boiled over through the overflow jug. The oil has always been normal looking and smelling, and I have never seen any steam or abnormal exhaust. Any ideas?