Any chance at all that alt+right could be getting hit once in a while rather than ctrl+right? That would take you to the end of the questionnaire rather than to the next item. I ask only because if it's unpredictable and relative infrequent, the proximity of the alt key to the ctrl key comes to mind. Possible?

Also--less plausible, but thought I'd mention it--depending on the version, ctrl+alt+right would take you through any/all questionnaire & files to the very end (e.g., in the case that you are not just running a single que file).