2000 Ford Escort IRRITABLE HEATER SYNDROME!

My grandma's car, the 2000 escort zx2 is the car that I drive back and forth to work and I leave early morning when its really cold and defrost is typically needed to clear the frost. The heater halfway quit working about 2 months ago. It works when it feels like it I guess. LOL It blows out hot air only when I am going 70-80 MPH and when I slow down, it blows out air so cold it feels like the A/C, but how is this happening, where is the cold air coming from when the engine is hot? The excess heat has to go somewhere does'nt it? I tried filling up my antifreeze, that did not work, like I knew it would not but granny insisted. I put a new MAF on it and took out the main relay fuse under the hood to reset the computer and the check engine light went off, the car started running a little better but still hesitated a bit and the heat started working great almost instantly even before the car got warmed up! 4 days later, check engine light came back on, heat stared blowing cold air most of the time, and the car started putting again. I have a warranty on the MAF but I don't think that should have anything to do with the heater working. I am going to order a new one and try it again but I really don't think thats the problem. I thought maybe the thermostat was sticking, and then I thought maybe it was the heater core, but the care is not overheating, although, before the heater quit working like that, when I used the air in the fall, water would drip on my feet while I was driving, but there is no water leaking into the passenger side floor board which is a classic sign of a heater core gone awry. I do all the work on this car, I've never had a problem figuring out what is wrong with it and fixing it, but this has got my puzzles so I thought I would leave it to the pros. Lol Please help me figure out whats wrong with it so I can be warm and not have to spray expensive de-icer on the windshield just so I can get to work. Thank you for all of your time and help!