Most probably processor issue. DW does not like dual cores. Use this tool for binding DW to first core:
http://www.activeplus.com/us/freeware/runfirst/
Or you could even use task manager for that. Right click on the process and choose 'Set Affinity' and then choose only one core.
AFAIK ATI is better for DW since it has dithering in 16 bit mode and DW only runs in 16 bit mode. I use nVidia for years so I don't know which ATI cards exactly have it, but some guys with ATI posted screenshots here and they looked 10 times better then what recent nVidias can do.