Why does my car die when I use any air vent setting except the floor vent?

I have a 2000 Ford Taurus and it dies when idling both in gear or parked, but--and I find this very odd--It doesn't seem to do it when I have the vent set to floor, but if I switch it to anything else, it sputters and/or just dies. It's been into the shop a few times and they can't seem to figure it out.