If you bought it used, I would expect you to at least ran WD Utilities to find out if there is anything wrong with the hdd. Then zero the hdd to make sure it is clean and doesn't have any bad sectors. The sporadic performance tells me either the hdd has bad sectors or highly fragmented. Also the CPU utilization is a little low, so make sure you are using the right interface mode, that is UDMA 133 or at leat UDMA 100.