Problem solved. Truly sorry for the trouble.
Turns out that Vizard couldn't render at the full 120 Hz (hitting only 60 Hz instead), because of a bandwidth problem that I'd forgotten had arisen in the past with two devices connected to our video card (see
http://www.worldviz.com/forum/showthread.php?t=1634). At some point in the last month, thanks to a firmware update on our projector, we discovered the resolution could be turned up (from 1024x768 to 1280x960). Unfortunately, I forgot to confirm whether the video card could actually handle that size at 120 Hz: apparently not. Thanks for the help!