How long had you been driving when this happened. Reason asking is your thermostat could have stuck in the closed or partilly closed position as the engine warmed up. Also the thermostat could do the same thing after it warms up espically in cold weather. First check the operation of the heater control valve, after the engine reaches operating temp. With the heater turned to its hottest setting check the inlet and outlet hoses on the heater control valve to see if they are both the same temp. If they are not then you can proceed in checking why the valve is not opening. Ther are other things to check like is the valve actually getting the vaccum it needs to work properly. The thermostat sticking could be the culprit but I would first check the heater valve operation . Keep in mind if the thermostat is old or sticking then you need to change it imediately. Even a new thermostat can fail causing catastrophic failure to the engine and more than often without warning. I only use Wahler thermostats but have had an occasion where it came appart and loss of one engine due to sticking while wife was driving. The heater fresh air vent could also not be working properly due to a bad servo that opens and closes the flapper door or even the inside heater/ac control panel not working which is another common inside heater/ac malfunction. First check the above things before jumping to this conclusion. Post back with results. Hope this will help Larry