It would help us to know if the blower is working correctly. If that is working as it should be when the trouble occurs then the air blend door system may be having trouble. Also make sure the coolant level is topped off so no air bubbles are in the coolant. Another senario could be the coolant is getting pressured up due to a headgasket leak and air is getting into the coolant system. A block check test will show that problem up.