check the seal on the cap to the coolant over flow tank. there should be a seal in there, and if it is cracked that could be the cause of the problem. from what i understand the seal builds up preasure, and thus does not allow the coolant/water mix to boil. if it is cracked, the preasure is going to be lower, and it will boil (which would explain the bubbles) hopefully this is the sourse of the problem, simple as replacing the rubber gasket on the overflow tank cap.
PS. never open it while it is hot read post below
[Modified by ChipGTi, 12:46 PM 10-11-2001]