Mike:
Perhaps it is overheating. Stuck thermostat, clogged radiator? Does the
expansion tank fill up rather rapidly when this happens? Water's volume
increases dramatically (250 times?) when it turns to steam. 
Another thing to check for is hydrocarbons in the expansion tank. My
friendly neighborhood mechanic used his emission tester to "sniff" for
these gasses. This would indicate a head SEAL leak. The seal is not the
same as the head gasket. Since your gaskets are leaking perhaps one or
both heads are warped causing the internal seals to leak combustion
gasses into your coolant.
Good Luck
Ken Lewis
kdlewis@juno.com
86 VW crewcab 
85 VW Vanagon GL
60 356 coupe

On Fri, 31 Jan 1997 11:57:52 -0600 "Michael D. Dower"
<m_dower@oz.plymouth.edu> writes:
>It seems that I either did not get all the air out or it is getting in
from some place else
>-mike             busted flat in cow Hampshire 
>