I've been having issues as well - system drops after 10-12 hours. I recently installed a couple 40gb hard drives and went back to DSM 4.1 as a temporary debug scenario - I had the system up for a week before the drops and I'm trying to isolate what changed. Not sure if 4.1 is the fix or not. When system drops I can still ping, but GUI and file transfer are gone. haven't tried port 9091