Check and see if it just shifted to the floor/vent setting by itself. If it did then you probably have a vacuum leak.
One other possibility is that the blower motor is going out, or the squirrel cage on the motor shaft is free spinning when the shaft heats up. Most motors have a set screw or a nut holding the cage in place but some are press fit. Any of these ways of attachment can free spin in the right situation.
After checking to the vent/vacuum I would pull the motor and inspect it and the squirrel cage.