I had this happen a lot with NASlite+ I think it is to do with the speed the drives are accessed, my old board only supported dma66 and it happened if I was transferring large files,you get an error about too much work at interrupt.
I have had a new motherboard and processor in mine since xmas and it hasn't "crashed" since, even transferring a 22gb high def movie, new board supports ultra dma 100. I have a monitor connected permanentely to my NASbox becasue of this problem, I wanted to see what had happened, your right at first glance everything does seem fine but the pc has frozen so only a power button restart will work.
My only suggestion is maybe upgrade to a board that supports ultra dma 100 I used a new MSI K4M board and an AMD XP2400+ processor about £75 from ebuyer but I'm sure any more up to date board will work
Best of luck
Wilbur