This may have been posted already, but I'm fairly new. My 1998 Blazer will blow hot air then cold air, no matter what the temperature setting is, or where the air is to be blowing. I'm sure this has happened to others, what's wrong and how do you fix it?

Check your Radiator, It's low on antifreeze.......

my rad is full, and clean along with my heater core. But my Blazer does it to a point too.

In my case it depends how hard I'm working the motor. If I'm going up a hill or stop and go traffic it stays nice and warm.
If I'm going down hill it'll almost blow cold air.
And if I'm just putting around at 30 for a while the air is just about right.

My thermostat is pretty new, so I think that it's acting normal. I've seen several other cars do the same thing

I would make sure that all of the air is bled out of the system. Do any of you hear gurgling behind the dash when you first start the car for the day and accelerate into traffic?

No noise with mine, and I filled my motor up with fluid through the heater core (filling it from the top hose)

I seriously think that it's normal in my case. With the little 2.8L Bigger motors don't usually seem to have this problem. I guess that they just flow more water through.