CAR HEATER TROUBLE

I've had a 1995 Buick Park Avenue. Mileage - 171,000. Engine size is a 3.8L v6 3800 series 2. Just this year my heater hasn't been blowing hot enough air out of the ducts. To get hot air I have to let the car idle for about 15 minutes with the heater on medium fan speed. Then when I start driving again the air doesn't come through as warm. Also when I have the blower fan speed on high the air comes out more cold. There isn't any leaks coming from the core or radiator. There is plenty of antifreeze in the radiator and in the access tank. No leaks on the passenger floor. The water pump was just replaced. I'm not sure if the heater core needs to be replaced or if it's the thermostat. Please help. Thank you