Things to try: swap out the Ethernet cable connecting your DNS-325 to your LAN and cable between your PC and LAN. To rule out your router as an issue, you can try connecting the DNS-325 to a dedicated switch (apart from your router). Although I don't specifically own the DNS-325, I keep my four ShareCenters (DNS-343/345) on a dedicated switch to isolate traffic. I've also found that adding the switch has improved throughput performance between my ShareCenters and PCs.
The latest numbers you are presenting are not unreasonable and are in the range of what others are reporting. I'm sure you'll find DNS-325 owners with higher throughput, as there are exceptions depending on network configuration and other variables.