When using GI and DR it is VITAL that you use a Pre-Pass, make sure this option is turned on and your buckets should come out the same intensity.
This could be caused by Window 2000 Service Pack 4 which currently causes corruption of MAX files.
Black buckets can also occur if you use the viewport lock in the render dialogue but have a different active viewport - it seems that the slaves render the active viewport not the locked one - make sure that the active viewport is the one you want to render. NOTE: This was fixed with SP1
Also make sure that all your slaves can access any map files - they must be on a shared network drive (or use the Distribute Maps option).