GM / General Motors: 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.

>I have done the following: changed the 

Well, ya gotta a leak....somewhere. Radiator, hoses, heater core,
freeze plugs. A pressure test should reveal the cause. Since the head
was checked for cracks/bad water jackets, could the gasket have been
installed wrong, or not torqued correctly? More detective work is
needed.

Good luck with the mystery.

Dave S(Texas)