I’ve done some optimizing (trashing preferences, etc.) and more testing. It’s definitely a problem in Cubase 8. I’m getting dropouts and glitches with just one track and no plugins. Cubase 7.5 is running fine on the same machine, same hardware, same drivers, same settings, etc.
I’ve got some sessions upcoming, so I’ll just stick with 7.5 for now, unfortunately. I can’t afford to have my clients worrying about my system.