AC blows from vent to floor to defrost intermittently
 

My AC on my 2006 Ford E350 is blowing air through vent, floor and vent intermittently. It only does this when the van is moving. It doesn't happen while idling. I have changed the Blend door motor and I am still having the problem. Also sometimes when cold air is being blown out of vent there seems to be hot air blowing from the floor. Please help.