Try this:
1 - Turn shadows and AA off
2 - Enter system manager (Ctrl + alt + del), go to the processes tab, click with right mouse on Sh5.exe and change priorities to "high" or "real time".
Also, I strongly suggest using the mod found here
http://www.subsim.com/radioroom/showthread.php?t=163847
as it changes lightning and reduces the impact on FPS (at least it works this way here)
Happy hunting
