1998 Malibu heater problem

I've got a 1998 Malibu 3.1 and the heater fan motor works fine, but sometimes (In the winter) it blows warm air and sometimes it is not warm at all. The fan always blows air though. Coolant level is fine. It does have a heat gage and it shows the motor to be running at the right temp. Any ideas would be greatly appreciated.....

Try back flushing the heater core.

Thanks.I will try that

sounds like you have a leak in the core....