The watchdog hardware (if you're not familiar) will reboot the computer if it locks up after a given time. The sp5100-tco has been often mis-identified and the module loaded as an intel i2c chip. Does lsmod|grep i2c-piix4 give any output? If so, look to see which driver the watchdog has loaded (lspci -k should do the trick, though you may have to sort through the output of that.) I've run into this same issue with an earlier AMD chip and an Asus Motherboard. Blacklisting the i2c-piix4 module solved my problem, though in your case blacklisting sp5100-tco should work, disabling the watchdog.