just to follow on from this, when the computer crashes, my mobo (evga p55 ftw) does the following beeps: 2 quick beeps, a pause, then 8 more quick beeps. i had a look on the evga website and it says:

8 Beeps - Display Memory Read/Write Failure Indicates a memory error on the video adapter. Try re-seating the video card. If it still beeps, try replacing the video card or the memory on the video card.

Have reseated gfx card and even swapped it for an 8800gt and i still get the same problem

Mate tbh this is out of my league. I'm tending to think that it isn't your videocard that is the problem as it happens with both the 5850 and 8800; unless both are faulty.

Could your pci express slot be failing under load? The reason I say this is because you don't get the problem on either card at 60hz, correct?

Maybe both cards are incapable of powering the 2233rz at 120hz. Like I said earlier, I've read of people with 5870's having problems so this may be the cause.

Can you enable it, but leave the speeds as it is? - might work out (I never leave anything on auto)

If that doesn't work, run CCC 10.6 or 10.5 and see if it works

Keep us updated mate!