Got the positional tracker working in the end, using a USB 3.0 card that had to be in a PCIe slot that didn't share bandwidth with any other slots, and also the tracker had to be the only device plugged into the USB 3.0 card (see:
https://forums.oculus.com/community/...-win-10-gtx970). Between this and the graphics card driver update, I think the nausea problem is solved. I might well update my versions of Vizard and Oculus as well, if other problems arise, but will wait and see...