Thanks for the suggestions, Mark.
The ZIF has a good connection and I even snipped 1 mm off the tip of the cable to move the contact points closer. No dice.
I made a custom build of PRS+ and got it working by reassigning the volume up to doMenu. (Thank you, kartu). Now I can switch books again without rebooting!
https://www.mobileread.com/forums/sho...64510&page=102