when you select the fan speed using the wheel the EATC does not control the fan speed based on cabin temperature - so you can forget the cabin temp sensor.
Based on the description of your symptom it sounds like the switch is erratic. I've seen blower motor resistors fail several ways, but never have I seen them erratically change speeds on its own.
Either way your top two things to look at are the EATC(and its build in speed wheel switch) or the blower motor resistor.
good luck.
I eat apples until my stomach feels much more full than its normal.