Remove the spare tire from the carrier. this will give you better air
circulation around the radiater. The "bubbeling out the over flow tank" has
me uneasy from here. A leaking copper sealing ring can let exhaust into the
cooling system as a frothy foam and allow overheating. This must be
eliminated as a suspect. Remove about 1 qt of coolant in the expansion
tank , to allow room for the foam in this tank,as opposed to the overflow
tank.Now you can have a service station  stick a emissions testing wand
inside the tank and check for carbon monoxide, or you can do this yourself.
Type in carbon monoxide into the archives(7/05) and there is a test there
with a  common household carbon monoxide detector that may let you do this
yourself. If you find exhaust in the cooling system, I think you can
disconect one spark plug wire at a time to see which cylinder is the
culpret.