I have an 1999 ford ranger and the heater control changes when you accellerate.?

while sitting idleing you can put the control on the floor vent and turn the blower on high and it will continue to come out at the floor but if you chsnge to another position defrost or vent it will not go back to the floor un less you turn the blower to the lowest setting. also if you start out with it on the floor and accelerate down the road at about 30 mph it will change to defrost by itself. so my question is, is this electrical van or mechanical problem