Artista - I think you may have hit on the problem! When I was finally able to get a good ride going last week I focused on several of those problem switchbacks - especially the ones that don't have a drop-off at the apex of the curve.

I remembered the advice here about focusing on the outside of the curve rather than the inside - and of course looking through the curve. It went much better, so that does seems to indicate I've either been under steering or trying to take the curve like I do on the road (hadn't thought about that).

I also figured out my problem with several armored crossings. I made myself ride them and found my problem hasn't been with the actual armored crossing, but the switchback right after them. This was good to finally figure out The switchbacks are much easier going downhill, so I can obviously ride them - I just need to keep working at it.