We have not had similar cases reported nor do we have a known issue with a system rebooting on its own when the network cable is inserted. Besides, having the "On-Board LAN" BIOS option is disabled is not normal. Hence, to begin troubleshooting, I'd recommend the following:
1. Restart your system into BIOS by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system.
2. If the problem continues, clear the CMOS:
Note: Make sure you leave the battery out for 20 min., at least.
Thx for the feedback. I haven't had the time lately to look into the issue and the strange thing is that the problem seems to have solved itself.
A week ago I turned the machine off and today I thought I'd have a look again and try your suggestion. Before doing so I turned the machine on, plugged the network cable in, and expected it to reboot again ... but it didn't. Just started normally. I haven't changed a thing. Not sure what the happened but appearently it needed the downtime ;-)