I've had the dual 800 since they were first released and have had only a few freezes, and those were associated with a sleep problem. However, I'm not much of a user of the sleep feature, and actually have it currently turned off.
I do believe there have been reports of sleep problems as you described many differnet systems, not just the dual 800. Anyway, Apples system updates have had patches to fix the sleep issue. But I have no idea which system updates they were in. You may wish to just make sure you ar at the most current version and then check if you still have the problem.
Tom N.