Faulty blower control

The knob you turn to speed up/slow down the blowers on my P38 is playing up.

If you turn it a click it will come on, and it will continue to increase speed for a few more clicks but then after so many it just stops rather than getting any faster. Then if you wind it further it just starts from the beginning again.

If you have a fiddle on with it you can manage to get it on full wack but it's certainly going a bit dodgy.

Any ideas as to why? Is it a known problem?

Mine does the same but since I keep it mostly on auto it doesn't give me much bother.

It's both rotational and sequential, i.e. after 5 clicks = on full, another click and back to 1, or low