you said the ic pump was working,,,,,i think you should recheck that,, I would remove the sprint booster and then hot wire the pump on (straight from the batt) then u have 100% eliminated the pump if it is good or bad,,, I say pump,, one,,,,the pump is worn out which under full power it can not pump enough water,,two,,,when the temp goes up(on sc water,not engine temp) the pump stops working but can come right back on with the release of the gas peddle, also if you are on the edge of the temp of the off switch the pump will go off and on within seconds of each cycle
USAF 1964 to 1968 jim