I found how to flush it using the wizardry of the search function.....of course my outside hose is probably frozen right now due to the cold weather

 

I am also having the same problem. I have replaced the Thermostat and Fan Clutch. Found a website today called heatertreater.net that claims the blend door is the problem. I have hot hoses in and out of the heater core and radiator. Tells me I have good circulation. Engine is definitely up to Temp.

 

This is the third thread on this exact same thing this week alone....you the search option and you will find answers quicker since they have likely been posted already....a lot easier than posting quaestions and waiting for someone to answer them again....
most of the others found that the cooling system and heater core needed to be flushed out....(like they should be every few years even though most of us usually dont, myself included)

 

I though the heater core was like a radiator....couldnt fluid could pass through it from one end to the other but still not be evenly distributed throughout the transfering channels of the core where the air is supposed to take the surface heat and heat the air with it?.....Just curious. what about leaves and such? could they also be blocking airflow in certian areas? never had mine out just wondering.
flushing the system out especially if not done in a long time is still the best first approach I'd think...