So Shane was right (as he so often is). USB 3 is NOT fast enough for UHD footage. I bought a thunderbolt cable and that did the trick. Thanks everyone.
I just unchecked “High Quality Scaling for Real Time Decode” in the Video Display Settings. It seemed to work for a moment, but then it went back to stuttering. Sigh.
Thanks everyone who commented. The solution we found was to update the drivers and firmware on our AJA 4K, then enable Main App and Host Audio within the Audio Mixer settings of the AJA Control Panel application.