I request some help with what has turned out to be a chronic problem of
overheating on my '87 vanagon.  When driving at sustained speeds on the
highway, everything is fine for a while.  Then the red warning light on the
temp guage starts to flash and the temperature starts to rise.  I pull over
and check the engine.  the coolant seems to be boiling and overflowing into
the "overflow" tank.  It fills well over the cap and starts to come out.  I
release some of the pressure and the engine cools down if I drive at lower
rpm, say 3000.  The temp rises again when going up hills or if I let the
rpms creep up.

In town, the red warning light often flashes, but there plenty of coolant
and the engine temp doesn't seem to rise above half way on the guage.  

I have been to the mechanic several times.  There are no apparent leaks, no
apparent combustion of coolant (actually it doesn't seem to lose coolant).
He has changed the thermostat, the thermostat housing and says everything is
ok, but it isn't.  It works for a few days than it acts up again.

Any ideas?  I have had problems with heads and head gaskets, could this be
related?  Is exhaust gas getting into the coolant?  What tests are
available?  Help.

Tom Z.