Heater Control issue

My 2000 Blazer LT 75K....has the fancy temperature contol system in it. It has always worked fine until now (it figures the Warranty just expired). I usually have the heat output down to my feet. Now all of a sudden every once and a while while I am driving or idling I feel the heat all of a sudden start to come out of the top vents. If I switch it to both top and bottom, then back to bottom only it does stop coming out of the top vents. But eventually it comes back out of the top as well even though I have the bottom selected.

Any ideas ?

Vacuum leak.

Check your vacuum lines going to the vacuum ball in the drivers side ack corner of the engine bay.

It is a round ball.

OK...will definately check that...

trace them to make sure any hasn't popped off the intake.

You might have to check the connections under the dash too.