heat/ac problem

So my car's ac had been acting weird, it was working on settings 1 and 4, but
settings 2 and three didn't change anything, it was still on the same strength
as the last number it was on. At that point it was just annoying not worth
fixing.. Then 4 days ago it decided that it wanted to be stuck on level 4 (high)
didn't matter if I turned the knob to off, it still stayed on full blast. I can could
change the temperature, but not the blower strength. By this time I'm getting
really peeved, it was making me go through gas like no other. Now yesterday,
it decided that it is going to choose when to turn off, and when to turn on. Still
the setting can be placed anywhere, but it will turn on high heat randomly for
a few minutes, and then just shut itself off..so this morning, when it was 31
degrees, it wasn't fun trying to defrost my car with such. Does anyone have
any ideas on what it is? I don't know if it's the fan speed sensor, the switch,
or the fan motor. Being that the fan works when it wants to, I don't think it's
the fan motor.