hello everybody,

my 9-year old desktop PC is having a weird issue.

this happens randomly, but most of the time, when i try to turn it off from the OS, it won't shutdown; it restarts/reboots instead.

motherboard : J&W JW-A870T-UC3
cpu : amd phenom II X4 955
ram : 2 x 4 gbytes DDR3
psu : seasonic 520 watt
os : dual boot, debian linux + windows 7

so when i try to shut it down from windows or linux, it reboots. so i have to force turn the power off from the UPS.
but sometimes it does shutdown the PC. it is randomly, so this confuses me.

which part of my PC is broken ? is it the PSU ?
thanks in advance.