HVAC intermittently switches from vent to floor: automatic AC

[Larryjb]

I'll set the AC to blow through the vents, and then randomly it will switch to the floor while the vent light is still lit. Pressing the vent button has not effect, nor does pressing the auto button. If I turn the system off, press the vent button and turn the AC back on, it will blow through the vents again.

Has anyone else experienced this, and found a possible solution?