Have you tried hitting alt+o to bring up the options menu and setting the max fps slider to 50 or 60? I put mine at 60 and I have never had an issue in any zone. I also heard someone put theirs at 30 to get it working. 60 has always worked for me tho...try both
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Also what OS you running on? I know WinXP and maybe Vista have a multi-core compatibility patch that helps this sorta thing. 7 Doesn't to my knowledge (doesn't need it).