It is possible that either your expansion tank is cracked or your pressure
cap has gone bad, allowing this to happen.  I have driven vans at sustained
high speeds with no problems.  The only other thing that could be your
problem is the dread head gasket leaking combustion gases into the coolant
thing, but check the tank first and cap (much cheaper).  I have these for
sale on my site if you need them.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16

New Phone (856)-327-4936
Shop: (856)-765-1583
Fax:     (856)-327-2242