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Reset the unit to factory defaults, disable all servers (ftp. AV, etc.) and reconfigure - enable just the essentials and then monitor it. The most common cause is one of the servers, but I forget which one.
Once properly configured it is rare that you need the interface anyway. Although as Fordem says, it you clean up the config it shouldn't be an issue.
I schedule torrent downloads almost on a daily basics, so the web interface is kinda important to me
Look into FFP (Google: FFP DNS323). The Torrent and Media players are significantly more stable and provide better interfaces. I download torrents daily and run for 200+ days without a reboot and even then it is usually for some other reason than stability.
It's a little unusual that you would have the DHCP server enabled on a NAS that has internet access - most people use their router as the DHCP server.
I got a meh transfer speed (1 min 15 sec to transfer a 1.75 GB file from NAS to Laptop). I will open a new thread for this soon