My system started doing this after upgrading my video card and moving to a higher level of the ATI Catalyst drivers, and eventually the game started crashing. It got REALLY bad after moving to Win8 (my PC playtime was still in XP until Christmas). After much testing, I discovered it was because my RAM timings were a bit too aggressive---even though it was using the SPD settings. After manually going in and adjusting them to slow the memory down just a hair, it smoothed out. YMMV of course, but it is something you could try if you are savvy with your BIOS configuration. If anything is overclocked (including if you have a special edition graphics card), may want to try undoing the overclock or at least scaling it back. You may be able to test it with something like Furmark so you don't have to keep logging into the game and all (the jitter happened when I ran Furmark in Windowed mode, making it much easier to find the sweet spot).