iMac 24" (early 2009) shuts down randomly

hi! reading forum years , now... registered because problem (ouch!) ... wonder if can help....

imac shuts down randomly since installed yosemite. happen when i'm doing something, opening safari, launching app... system doesn't shut down if don't touch it. made investigations....

- re-install mavericks , snow leopard , problem remains. not related yosemite @ all.
- apple hardware test (the long one) says there's no problem piece of hardware (the imac didn't shuts down while performing test).
- there's nothing plugged mac, problem not related peripherals.
- there's no pattern system log. last messages recorded before system shuts down different (no errors).
- temperature registered in sensors fine, fans working properly.
- have ups unit current should nice , protected.

so... can shuts down fault of power supply unit? can switch on mac after shut down without problem. there's way test psu before pay new 1 (expensive part!)? idea of different causing trouble?

thanks!!

 

maybe silly suggestion, did check power cable set on imac (not on ups).

 

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