Weird Heating Problem

Ok, I start my 95 mercury mystique with a 4 cly up and it seems to heat up reasonably fast. Now, if I just let it idle (i.e. a cold morning) the engine will overheat and I won't get any heat inside the car. If I start driving and keep it at around 2,000 rpms, then I get heat. Also, running at this rpm (2,000) will cool the engine down. So after just a minute or two of going down the road(after it's idled for 10 mins) I'll get heat, but then as soon as I come to a stoplight, the heat will disappear and I'm back to cold air. Since I've replaced the thermo and the coolant level seems to be correct and the fact that I can get heat, just not consistent, it seems like a water pump gone bad. I replaced the pump about 75K miles ago. Could it be the water pump? Thanks for the help.

it could be a problem with the blower or physical blockage of the ducts. the control valve door might not be opening. the heater core might be starting to go. it could be the water pump, with out the right tests being ran, its hard to say what the problem is. these are just my thoughts.

hope this helps

Terrell