I have no experience (yet) with using Reaper for video, but I think for those who do, you've narrowed it down to being app specific, given that a) it worked well before and b) that you can play back videos on your system in another app (I'm assuming).
My first inclination is that a software update changed a setting or that your preference file is corrupt. A long shot is that a stick of RAM is acting up only when your system is being pushed toward a higher performance.
Just some thoughts. Good luck.
