Do you have Anti-Aliasing turned on, or a widescreen resolution selected, by any chance?

On my system (A64 X2, 7800GT SLI), the game does this:

1) Intro flickers to black with choppy sound if a widescreen resolution is selected. Normal 4:3 aspect resolutions are okay.

2) Cairo station: while crossing the moving platform just before getting to the Covenent bomb, the game crashes consistently unless I run a 4:3 resolution and disable anti-aliasing. However, even without crashing, fighting the elites in that bomb area is incredibly choppy (~20fps)...I usually get 50-90fps even during normal heavy firefights.