Ok, so here is the important question - is the car overheating, and therefore boiling over - OR - is the system leaking, which doesn't allow it to maintain pressure, and then subsequently overheating and boiling over.

IT WAS LEAKING! There was a pin hole in the cooling hose that goes from the water pump down into the fan housing (90degree 4" long hose). This was leaking (only under certain conditions, but definitely when the car was turned off after a short drive) and then allowing it to overheat and spill (in addition to the small amount coming out of the leak).

I replaced this hose last night, and have seen no leaks or problems since.

Thanks for everyone's help.

-David
92 500sel
86 560sel
85 535i
82 Qporte