2005 Colorado Heater not always have air flow into car.

I have a 2005 Colorado that has an intermitent problem. Sometimes but not always i feel the heating system blowing from the defrost mode or the vent mode or the feet or lower mode. The blower has all the speeds and always works as you can hear it running. The problem is you might have blowing for a few minutes or hours and then suddenly you start getting cold because you can not feel any air coming from any vents etc. It almost seems like all the doors for the vents are closed at the same time not letting the air pass through? I am sure the heater core has hot water. I mentioned the fan always runs. The controller sounds like it might be trying to reroute the air flow but still no air flow. You can start the car the next day and it will work, go figure. The thing is when it does not blow the air almost sounds like it is trapped looking to excape? Just a thought. I need some input on what needs to be corrested here. Is it a vaccum problem? Any or all help is needed since my warranty has came and gone. I know if i bring it in to the dealer they will own my bank account since it sometimes works and is an intermitent problem. Please advice if any body out there had the same experince or know the fix here.
Thanks in adavnce
Al