Assassins Creed Revelations screen freezing/stuttering

Assassins Creed Revelations MP Game Stuttering AMD Phenom IIx4 840 3.2Ghz Asus M5A78L-M/USB3 4GiG AMD 4850/512 WDC 250G SATA2 All drivers updated and freshly defragged. Ok, not the highest end system but not the slowest either with 4 cores and i certainly dont want to see stuttering on LOW SETTINGS. I mean low as in everything on low. I could play fine for quite some time and then all of a sudden it starts stuttering out of teh blue. No other games do this.