I had a somewhat similar problem... except my heater blows hot air after that initial warm-up period. I think it's because the cold weather makes each element of the cooling system cold - hoses, radiator, heater core, block, etc. - and so every one of those things is making the hot coolant colder... therefore, even though the engine is at temp, the thermostat opens and BAM, the coolant is instantly chilled by 20-degree hoses and radiator. The temp sensor must detect this... for a while I thought I had a busted thermostat, but it turned out not to be that at all, because after driving for a while it DOES reach normal operating temp and stays there.

Now for a solution... if I turn the heater off while I warm the car up - let the needle get somewhere near normal - and then drive with slightly higher revs than normal, it heats up pretty normally. The other solution is to let it warm up for a longer time, and let the hot coolant slowly warm up all those cold engine parts which have been chilled.

Just my two cents...