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Naughty Boy
1 Copper

Dell Optiplex 780 Keeps restarting without any notification.

First of all sorry for bad English.
previously i bought a dell Optiplex 780 SFF Out of warranty, With spec

Dell Optiplex 780 SFF (Small from factor) 

Intel Core 2 Duo 2.33 4mb cache (E6550), 2Gb ram, 80 Gb SATA HDD, DVD Super Drive,

I updated its BIOS from A07 to A15, it worked fine for 2 weeks after that is started restarting. from the very beginning it restart 1 time a day after 3 days it did restart for unspecified timing. some times 1 restart over a tight many times after every 5-40 mins. i don't know whats the problem with it. i changed Windows many times, i diagnose HDD but there was no error on hdd, Changed RAM (worked fine for TWO days) but again the problem continue, I checked CPU temperature but that was fine about 27-42 Degree C , I updated video driver, I Degarded the BIOS (Checked diff. versions A07-A15), One most Important thing there was/is no BSOD (Blue screen of death) Before are after restart.

I have doubt about Power supply i didn't change it yet, is it because of bad PSU? Help me i'm in big trouble?