Heater

I had this problem once, turned out to be the lining inside one of the heater hoses that take coolant back to the heater core had split and would 'close off' the flow through the hose occasionally...replaced them both...fixed'er...

First, check the coolant level and condition. The coolant level in the radiator should be at the bottom of the radiator neck and above the cold mark in the overflow tank. The coolant should not be rust or milkshake-colored.

If the coolant level is low, top it off with the proper coolant mix.

Then have someone in the car move the temperature control from cold to hot and observe its action on the heater control valve on the firewall. You will see a wire cable moving an arm on the heater hose. Make sure the cable moves the valve to its limits.

Then check the thermostat operation by removing it, placing it in a pan with water and a thermometer, and heating the pan. Watch the temperature at which the thermostat opens, it should be around 140 degrees F.