It can be hardware related, but trying to 'prove' this may be hard.

The 2850 has dual onboard NICs. Do you have the 2 teamed? If not, consider doing this. It serves 2-fold; if you indeed have a network link issue, the other NIC will be able to keep the link up so you don't have the 'downtime', and it helps you identify that the problem is the link (could be the nic, the cable, or the port on the switch).

If you already have them teamed, check for a driver update, but a complete loss of network connectivity would then be less likely to be hardware related as it would mean you're experiencing dual failures simultaniously, which isn't impossible, but significantly less likely than for instance a dated driver.

Even though you may believe your network hasn't changed and 'suddenly' the server started losing connectivity intermittently, you have to keep in mind that servers still get frequent updates for antivirus, firewalls, Windowsupdates, etc. So keeping your drivers up to date is important as well, and it's very rare for Windows to update these for you (I've seen driver updates available as optional updates on XP/Vista, but don't remember seeing these on server OSes).