318i Heater problem

Hi guys, i am having some problems with my heater, it does not blow hot air in the low speads, if i am in the high way and driving like 40-70 mph then i can feel the hot air, but when i hit the traffic and go below 30-20mph the heater starts to blow colder air, even though the car temperature is showing normal on the cluster. the car has no problems warming up. I was thinking it is thermostat, which i am planning to replace this weekend, but again if it is thermostat how come my car worms up normally??
any ideas what else i should be looking for? did anyone experience the same thing? how did you fix it?
Specially now it's cold in outside i want to get this fixed sooner.

thanks

Sounds like the thermostat to me. As to normal warm up, if the "T" stat has failed open, your car will eventually warm up due to engine heat climbing, the "T" stat just restricts coolant flow to help warm the engine quicker , and as the engine temp climbs, the stat begins to open until eventually it is fully open (condition your car seems to have). Good thing is if it is the stat she has failed open and not closed.