Not fixed as of 2/13 patch. I run dual monitors and had several bouts where both screens went black for about 5-10 seconds. Happens more often on Twisted Treeline than Summoner's Rift for me.
Since the 3.0.1 update, often when I get engaged or a lot of flashy spell effects are going off (I noticed it pretty consistently when MF would hit me w/ the rain of bullets), the screen stops updating properly. I get about 5 seconds on avg. of not being able to tell what is going on due to screen not updating/stuttering terribly/going black, and sometimes there are some weird killscreen-esque artifacts (like a tiny box saying "FIRST BLOOD" 20+ mins. after actual first blood). It looks a little like when a video card is overheating, but I've monitored my temp and that is not the problem, nor are any other games causing similar issues. Once it even hard-locked my computer and required me to turn off PSU and reboot. It also tends to cause whatever window I have open on my 2nd screen (usually Chrome) to go black until closed/reopened (minimizing doesn't fix it).
I've done a clean install of new graphics drivers, and uninstalled/reinstalled Adobe Air and League. No progress. It is making the game absolutely unplayable, which is a real bummer since I'm having a lot of fun, and have dropped a decent amount of cash into it.
I'll try to keep the following list updated w/ other people's input in this thread.
Additional Info / Summary:
A video showing an example of the problem (posted by Moxxi Murdercute).
May be related to Nvidia 8800 series (others w/ this family of card encountering similar issues).
Causes problems on 2nd screen in two screen setup.
Updating to newest or even current beta graphics drivers does not help.
Uninstalling/reinstalling League & Adobe Air does not help.
Problem experienced in both Twisted Treeline and Summoner's Rift (and other maps by other players).
VSync set on graphics card control panel does not help.
Happens most often when spell effects go off (esp when getting hit by enemy spells).
Champion played does not seem to affect it (encountered every game, on many unique champs).
May not to be happening on PBE (per Doctor Bumble; I have no PBE account).
Riot, if there is anything we can do to assist further in getting this fixed, please let us know. Thanks for the great game, and your hard work.
DxDiag.txt is attached.