the heater 'temp sensor' and the 'heater valve' would effect the air temp in the foot wells also, which doesn't seem to be your concern. it sounds like a ventilation door 'stepper motor' or control unit problem....these 'stepper motors' ARE problematic on these cars and can actually break off the heater box. unfortunately you'll need to see someone VERY familiar with 740i's and the IHK system to properly diagnose it.