Well, yes, things are getting a bit worn and the know slips around on the post. Fortuantely, twisting it CCW does move it through the various positions.
The knob can also be easily pulled off of the post (did that once and found the brass bushing).
Sounds like the post (white soft plastic) may be getting worn so getting a new know may not be the solution.
I can't find my shop manual so this is the best place to ask the probably dumb question: Can the post be removed (i.e., it's hold on with screws to the actual rod that controls all of the vents' flaps) or is this a major undertaking (remvoe the dash and all that)?
Alternatively, should I just buy a new knob and bushing? Should I add a brass shim to the post to ensure that everything locks up for rotation.
Really stupid fix is obvious -- put goop on the post and glue the knob on permanently......
Snowing in Toronto..... it's a Winter Wonderland!!!!!!