Believe it or not, I just had this same thing happen in the RJ, and it was something very simple. So even though there are a couple other reports of this on the forum now, I have to ask: was your speed brake extended? I had apparently bumped my speed brake lever at some point, and I didn't realize it until I noticed pitch oscillations at cruise of approximately +/- 500fpm, just like yours in the video. Retracted the speed brake, the problem went away.