Just throwing out oddball ideas, since most of the good ones are taken <img src="/forums/images/graemlins/pfft.gif" alt="" />, but are you sure there isn't some sort of blockage in the overflow tubing that isn't allowing the coolant to be drawn back into the radiator? if there's a bit of junk in the line that's blocking the return it might just be the normal action of your coolant's expansion over several runs isn't able to reverse. The draw back into the radiator isn't that strong, a little bit of junk between the cap and the tank may be enough to gum up the works.

James