Heater Blowing Cold in 2000 blazer

I'm looking to see if anyone has any ideas about what might be causing my situation. My 2000 blazer LS used to blow some of the hottest heat in the winter of any car I've ever been in, then two winters, it started blowing cold mid winter, I had a mechanic "blow out the core" which caused it go back to blowing hot again. Then last winter, did the same thing, only this time, blowing out the core did nothing. For the rest of the winter, it blew cold.
The odd thing is that it is getting slightly cool again (40s-60s) and now its blowing hot again, but I am sure that once it gets really cold (0s-30s) it will blow cold again.
 
Any one have any ideas? I have a new son, so I need to fix it for good this time, can't have a newbord riding in a car at 15 degrees.

Thanks.

replace your themostat  its a a cheap fix!! good luck

Thermostat, I'm afraid is only a temporary fix. Your temp gauge should be right around or slightly below 210. If its not, allow ample time for it to heat up. If its blowing cold at running temperature, you are looking at a heater core replacement.
Before you do that, however, it would be a good idea to do a full engine cooling system flush. Radiator, heater core and all.

..we had this on my boy's 2000 LT. After reading lots on the Internet I checked for a gurgling sound coming from the heater core under the glove box, which we had. Long story, it had an air-lock in it, caused by a crack in the radiator that pulled in air as it cooled. I replaced the radiator, thermostat and radiator cap, and the carefully put in coolant with the car jacked up on the side keeping the radiator cap opening up on the high side..let all the air bubble out.
Has been ok ever since.
Earl

My heater will run you out of the vehicle... I had a problem last year and a flush took care of it.

I had the same problem in my 2000 S-10.  Your antifreeze (dexcool) which is the worst thing ever invented is jelling.  If you add regular green antifreeze it will jell to the point of serious damage to the cooling system.  This jell would clog my heater core and also collect in the bottom of my radiator.  I would hook the heater up to my garden hose and reverse flush the core and that would work until the next time it would clog.  the way I finally fixed it is by replacing the radiator and hoses.  (my radiator was so clogged my truck would overheat).  From then on use the green antifreeze that is compatible with both colors of antifreeze.