Climate control cycles from cold to hot repeatedly

1999 S-10 Blazer with auto climate control. It goes from cool to blowing hot, then back to cool, then back to hot, ect...
What is the deal here?
I've read about the blend door and stuff. But this doesn't seem to be just a blend door stuck open. When AC is off, and it is on heater, it still does the same thing, but, it just doesn't get as cool.
Any ideas???
Chuck

I know there are lots of posts on this. It's most likely you are low on engine coolant and there is air in your heatercore.

Since it's cold outside I will guess that you are turning on the heater and having this problem. I had the same problem and this is what I did

1. Check coolant level. Mine was low. This keep the heat constant but not as warm as in the past.


2. Check engine temperature when warm and replace thermostat if too cool.

So the engine temperature was around the 160* mark, So I replace the thermostat and it pumps the heat out again.

3. replace radiator cap.

I did this because I lost coolant. Everyone says the OEM cap is crap and leaks.

I will check the coolant level, but I don't think that's it? Actually it started doing this with the AC on. My wife always has it on 70* or so. It hit 80* outside last week and the blazer has the auto climate control, so it kicked on when she was leaving work. It's like someone has their hand on the temp knob and is rotating it from hot to cold. I was wondering what would go out to cause the automatic controls cycle back and forth?? I'm just guessing here!!
Any other ideas other than the coolant level?
Chuck

I know I posted this a long time ago, but I fixed it and just wanted to update incase anyone else was having trouble.
If anyone has trouble with the vents not closing all the way, or cycling from one to the other, there are vacume lines behind the glove box that can be accessed by pushing the glove box stop in and folding out the entire box. I had one just simply come off. It doesn't allow a good constant vacume on the system. Put it back on and wha la. It's fixed.
Hope this helps someone. We drive 20 miles on a dirt road going to see her parents, out in the boonies on the ranch. (10 miles to and 10 miles from on dirt road) It's pretty bumpy, so I guess it just vibrated off.
Chuck