I noticed when it got cold out the car would take a while to heat up?

I am having a hard time finding what is wrong with the car I just bought. I noticed when it got cold out the car would take a while to heat up, so I replaced the thermostat, and everything was great for about a week. It heated up fast and the heater worked great. About a week later the car takes about 10min. To just get above the the C mark on the temp gauge. When I drive it just a few blocks it drops back down below the C, and the air gets colder. I looked at the fluid and it is fine, and I even replaced the hoses, and I replaced the thermostat with another, thinking the first was faulty. The antifreeze is not leeking out or in the car, and the water pump was replaced 30K miles ago. Please help, I am running out of ideas.