I booted into Windows on bootcamp and it wasn't flickering on Windows. I then ran FSX on full settings, streamed on livestream and played Wallee in 1080 with VLC player. My cores all hit around 84C and no flicker occurred.
I'm back on OS X now and it's not flickering at the moment. I called Apple and opened a ticket. They told a case number and to call back to run some sort of diagnostic once Time Machine was done.
I'm at a loss here now. Was it a software issue after all? Did it only occur when the CPU / GPU was juggling lots of things and therefore will happen again? Should I send this guy into an Apple repair shop? Blahh
You're doing everything right, and judging from the echoes others have posted by now you may even be illuminating an unusual but widespread issue.
There's no point sending the machine in for repair until a fault is spotlighted. Right now it's not clear what to fix, and most likely you'd be without the machine for a week or more, only to receive it back with "No Fault Found".
My wild guess at this point is something going haywire in the power-control logic, which is indeed in software/firmware. Some combination of inputs (CPU/GPU temperature, fan speed, processor load, ambient light, time of day, phase of the moon... who knows-- and not necessarily extreme values) results in some sort of bogus logic state, and ergo: display blinks out, etc. Apple's power-control logic is unusually sophisticated and fine-tuned to the hardware, and with sophistication comes the opportunity for subtle bugs. The fact that you didn't observe the problem when running under Windows (which can take only a generic approach to managing power on the Mac) is suggestive, too. Not quite a slam-dunk, but illuminating.
(I presume you're NOT using some utility like smcFanControl. If so, let us know, and then remove it, please.)
Keep it up. If the folks here on the thread could try different tricks that might identify one or more causes, who knows, this thread might just serve to crowdsource the clues that Apple needs for a solution.
And keep backing up!