I've noticed mine flickering a little bit, but it doesn't happen all that often. I never used it stock, so I just figured it was an issue with the Gapps ROM. Again, it doesn't happen often, so it doesn't bother me too bad, but just to make your mind at ease....you aren't the only one to notice this.
No, this is a different item. Like you, I turn off the auto-brightness too. I haven't nailed down when it happens, but it's just a quick bit of feint flickering for me, then back to normal. Kind of hard to explain I guess. I've noticed it a couple times a day since I got mine last week with pretty heavy usage.
My thoughts exactly. The more I read some of the forums, the more I regret my decision of getting the HDX on sale vs the Nexus 7....I was looking at the great specs and not thinking of more, but last night showed me the potential this thing has when I installed the AOSP ROM. Not perfect, but potential. Liking AOSP over the Gapps ROM despite not having music and video.
While reading yesterday's XKCD "What if?", I was able to reproduce the flicker reasonably well. It showed up frequently on the white theme of "Press", but not the dark one (or I simply didn't notice it as much). I did a quick video showing it the best I could. Sorry for the shakiness. The primary time-frame it happens is between 12-16 second mark.