1997 Ford F-100 Heater goes cold then overheats

The heater started going cold, then it overheats and blows coolant out pressurized tank. When heater starts working again, it cools fine and runs great. One mechanic replaced thermostat, the other replaced water pump. I picked it up and it did it again before I could get to work, mechanic picked me back up and changed out the thermostat, but it did it again on the way home. Ran fine after I pulled over and the heater started working again. He tested with the green to blue chemical for head gasket leak, but was negative. Any ideas? Another mechanic keeps saying "blown head gasket in early stages". Would you replace engine or do gasket job? Someone else said the catalytic converter could be bad....help.