Guys/Gals, first off thank you very much, a lot of good comments and suggestions here. Having said that, I've tried 95% of them, to include checking the Rear D hanger alignment (dealt with that insidious problem before, not fun).

The gear in question is the seventh from bottom, fifth from the top (#17T) - apologies should have included that in the original post. As the cassette is an 11-25, it's also not associated with the jump (i.e. 17 to 19), it just doesn't like to shift from 16 to 17. (The gearing runs 11-17, 19, 21, 23, 25)

My suspicion is it's the gear itself, maybe there's a mar on the step of the gear that isn't engaging the chain correctly.

My other theory is much like many of you already mentioned, an issue with the cable.

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"It's easy to be hard, it's hard to be smart"