I've tested a pristine 4.6.0 system, so it's definitely something that
went in during the merge window.  The flicker isn't continuous, it's
periodic, with an interval of something like 2-5 seconds.  It looks
like an old analogue TV going out of sync and then resyncing.  I've
attached the dmesg and X.org log below just in case they can help.  I
might be able to bisect this next week, but, unfortunately, this is my
current laptop and I'm travelling this week.

James