How frequently is yours acting up? I noticed the flicker several times yesterday. Mine only had the one episode today (albeit a pretty severe one).
My theory had been that the flicker would appear in periods of heavy use. However, I tried to replicate the problem this evening and couldn't. I used the GPS, watched some videos, made a 30 minute phone call, watched some more videos, and visited some web pages. About two hours of continuous use and no flicker. That is why I am kind of on the fence about sending it back right away.
Mine was doing it almost non stop after I set it up yesterday (and during and after a recovery to try to fix it). Today I've only seen a little blip here or there, but I also haven't been using it much. Same as you I've tried to replicate what causes the problem, but haven't had much luck. The one thing that I did do when I first got the phone was get it warm from charging while syncing with iTunes. I'm wondering if that has something to do with it. In that case this is probably a problem with the CPU/GPU.
In addition to the flickering issue I've got a big dead pixel (might be something in the screen actually) and 5-6 stuck pixels. I'm getting mine replaced even if the flickering goes away.