1993 Mercury Sable heater does not work all the time

1993 Mercury Sable mileage: 71,000. The heater would blow hot and cold at different times. I pressure tested radiator and found a leak in the intake manifold. The heater gargled at times when low on water. I repaired the manifold leak, checked the compression (all six at better than 150 lbs.) Have a new thermostat and radiator cap. Still the heat comes a goes and the heat gauge goes up high at times.