The dog and I went out for a steep offroad knockabout with the S3X hub.
I pulled the splined 15t cog off, and substituted a Surly 17/21 dingle cog. I intended to use both gears, but ended up sticking with the 42 tooth ring and the 17 tooth cog.
I had some slipping in the lowest gear that seemed to be related to shifter slippage. When it happened, I noticed the cable was very slack in the high gear. I tightened up the cable (a simple twist of the connecting rod), and the problem seemed to disappear.
More research is in order.