I had an 83 WB that did this when the gasket let go between the cylinder and
the cooling system.  It never leaked, but would blow the coolant out when the
gas from the cylinders would blow-by.  I drove it from NM to MO by slowing
down to 55-60 MPH from the 70 I had been driving, and bled the cooling system
in the mornings before I started that day's drive.  Once or twice I had to
bleed it during the day.  By watching your temperature gauge closely, you can
see when it's about to blow.  The needle will start to rise very quickly and
with the practice I got, I learned to catch it.

There was some minor damage to the head that had to be lightly machined to
correct.  Otherwise, it was the normal  waterboxer gasket replacement.