I have had mixed results with SheevaPlug. I bought two plugs. The first plug I bought last August from Globalscale and right from the start it would reboot spontaneously after about 15 to 90 minutes, never staying up for more than a couple of hours. That plug was replaced (GS got New IT to supply the new one). The second plug (not the replacement, which is sitting in a drawer as a spare) I bought at the end of last year from New IT and it started playing up yesterday. It runs debian from an external USB disk and has been stable for about 3 months running exim, dovecot, nfsd and samba. However since yesterday it has rebooted 8 times leaving no trace of any problem in any of the logs (/var/log/{syslog,messages,kern}). The plug sits behind an APC SmartUPS so dirty power should not be an issue.
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I am surprised to hear that 2 plugs have played up, as I have found them stable. Just thinking out of the box here, is the plug faulty. or is this a result of attacks. If the plugs are visible to the internet rather than hiding behind a local network, then I can think of 2 reasons for problems. You could either be having DOS attacks, or exim could be being flooded with spam.