The drivers side vents on my 2004 Chevy Suburban randomly blow hot air sometimes if the a/c is on or not. All other vents in the car blow cold air when a/c is on and never blow hot air unless the heater is on. This happens at random times. Sometimes everything will work fine for weeks and suddenly the two driver side vents start blowing hot air. If I turn off the engine for 5-10 minutes they will usually resume blowing cold air until the next random time hot air start blowing again. Any ideas what causes this?