91 Pontiac 6000, Weird stuff going on.

I hope someone can lend me some advice before I shell out a bunch of cash to a mechanic. My car has been having cooling system problems. It continually bubbles back into the overflow after it has gotten hot and when it is cold outside there is a lot more white exhaust than seems normal, then after driving it for awhile with the heater fan running it starts blowing cold air and the temp light comes on, checking the overflow after this has happened has revealed low coolant, but it is fine when it is warmer outside. I have done the following: changed the radiator cap, changed the thermostat, changed the water pump, checked the head for possible cracks and replaced the head gasket. As you can see I have already put some coin into fixing it and I am still having this problem. Please help.