>   FIRST the pressure drops precipitously, and THEN the coolant level
> starts to go down ... FINALLY the temps start to rise (provided the
> temp sender is submerged in what water remains).
 
Just a brief comment on this part.  Coolant level sensors are of almost no use in this application.  It is commonly assumed that the temp gauge reads low as soon as the sensor is no longer submerged.  Not true.  It only drops after the coolant is all boiled away because steam will heat the sensor.  If you haven't caught the problem long before this, it's too late anyway.  Tracy