Project Slowing Down and Distorting

I still have Cubase 6, and recently when I’m playing my project, it slows down and distorts and then speeds up as if to catch up to where the playhead would have been if it didn’t glitch. I’m using and AUDIOBOX USB and the respective audio driver in Cubase. I’m tried changing the block size in the interface control panel but it doesn’t help.
Anyone have any idea on how I can fix this issue?