I have a NAS (iomega home media 1TB) plugged in router using 1Gb ethernet cable. Occasionally I lose access to it, I can ping it by static ip, nmap to see all open ports but can't telnet to it. Also http server doesn't respond. debian 5.0 there, arm processor, no iptables.
Sounds almost like the unit has something faulty causing it to die, or the switch (or port on that switch) that it's plugged into is losing connectivity. If you can switch network ports, try that first (the switch may be going flaky).
Test the cable to make sure connectivity is decent. Could be an edge-case where it's not working properly or making proper contact.
Last I'd try contacting support to see if the nas still under warranty for a replacement.
Yes, I know you said you can ping it, but I've had switches that had general flaky behavior that made no sense in their failure until I replaced the switch (and AP's too). Just something with hardware failure made them go wonky.
Could you describe the problem in a little more detail? Are you able to ping it 100% of the time, but unable to to connect via telnet or HTTP periodically? Or is it that periodically the thing just goes offline (no ping, telnet, etc)?