The two actuators in the air system are prone to failure. The actuator that controls air temperature can either fail, or the flap binds (mine binds). It can be intermittent. I finally disconnected the actuator control wire (near the front fuse box), moved it by hand to the full heat position (there is a big air hose in the center front of the car you disconnect to get access), and installed the Tony Waa heater core coolant valve kit (search for it at this forum). I also made my own electronic switch to control the valve from the regular heater knob. Note that when the flap is in the full heat postion, the air flow is over both the air conditioning core and the heater core. Hifiandmtb, this really seems like the problem you have.
The other actuator controls which vents the air goes to (floor, upper, defrost).
Markirlanda, you may have both of these problems. When it happens again, test that the vent actuator is properly switching the airflow, then see if the hot air follows the vent selection. If the vent selection works properly (and not just partially - that is, the flap doesn't fully go into a postion), yet the hot air still comes out near the floor, then I don't know what your problem is.
Of course, the inner sides of the chassis in the footwell sometimes get hot in the summer due to the coolant lines running through there.