Q: Car heats up but only blows out cold air and the temp. Goes up then back down. Radiator is full of anti freeze

The engine temperature needs to be above 180 to 190 degrees before the thermostat opens and you will then have heated coolant flow to the heater core. If your engine does not keep a steady temperature then the thermostat may need to be replaced. If temperature stabilizes at about the 190 degree level then you should check the two heater hoses going into the firewall into the heater core to see if they are hot. If they are then you need to check inside temperature control actuator on the a/c heater box inside to see if it moves to increase or decrease temperature control. Replace actuator is it does not move with temperature control.