Pretty strange behavior allright.  I see that the Bentley manual does list
this condition under trouble shooting and suggests that it may be caused by
the master cylinder although I don't personally understand how that would
happen.  If it was a seized caliper it would probably only happen to one
wheel unless the car has been sitting unused for a long period of time.  Do
all the wheels get hot or just one??  It seems like you have ruled out the
antiskid controller and the master  cylinder so the only thing left is the
vacuum assist.  You might try removing and plugging the vacuum line the next
time it happens to see if that fixes it.  Just remember that you won't have
the power assist with the vacuum disconnected.
Good Luck and please let us know what the problem turns out to be.

I am experiencing a problem where the brakes lock up after about four
to five miles of travel.  I was told that the problem could be
resolved by replacing the master cylinder.  I did, but the problem
continues.  The only way to get going, is to release the pressure via
the bleeding valves.  This works sometimes for an eighth of a mile or
sometimes up to ten miles.  I removed all the relays from the ABS unit
with no luck.

Is there someone who has experience this problem?  How was it resolved?