L4-2.4L,My AC does not always work. If I fiddle with the blower knob I can ususally get it to work on the lowest setting.

My AC does not always work. If I fiddle with the blower knob I can usually get it to work on the lowest setting. And if i fiddle with it some more once in a blue moon I can get the higher settings on the knob to work. But they never work on the exact spot they are supposed to. Usually they end up half way between to setting numbers on the dash and a small bump can knock it out of wack. Also on the rare occasion I do get one of the higher settings to work if I shut off the car then restart it it usually stops working again. Can you tell me what I need to fix it?

My car has 104000 miles.
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