Erratic Air Conditioning

My 2001 330i SE Sports has started to suffer from erratic a/c. When you switch it on, it often doesn't start but if you press the on/off, auto,
Up/face/down, fan speed (as far as I can see at random) it will sometimes start. It sometimes then goes off after a short time but sometimes it then runs properly.
Does anyone have any suggestions about how this fault can be rectified (other than the obvious remarks about take it to the garage)?
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