My '84 Vanagon camper has this problem where every 30,000
miles or so the head gaskets begin to leak, causing gas buildup in
the water  cooling system.  This first appears as a rising coolant
level in the overflow tank next to the oil dipstick. Next the
dashboard heater quits because it fills with gas.  Finally (about
2K miles later) the leaking gas literally blows coolant out of most
of the cooling system and the temperature sensor begins to signal
imminent meltdown!
     This has happened to me three times now.  I travel alot, and
so have talked to mechanics across the country. They imply it is a
common problem, that VW doesn't really admit it exists, that there
has been a service bulletin on it, that the only thing you can do
is remove the heads and install new seals ($2K!).  Some think it is
due to improper coolant, so I have used only VW's brand since I
bought the car with 33K miles on it in 1987.
     Does anyone know how to *FIX* this problem or prevent it from
recurring?  Any related information or advice would be greatly
appreciated!        George (gruff@abacus.bates.edu)