Yes, it was happening while I was using it. I first noticed while I was playing Words by Post.
I've always had battery saver mode enabled in both my 810 and now my 920 but never saw this happen before. I changed the brightness to low and turned off automatic and it stopped doing that. Right now I have low brightness with automatic back on and it's fine. Weirdly, it seems I'm suddenly having the problem when it's set to medium with automatic. I also wonder if it has anything to do with the fact that I charged my battery three times today (once not to full capacity).
Ugh, while I was typing this on my phone, the screen suddenly went quite dark... But at least it's not going dark and bright alternately like it was before.