PC problem

[mirosbee23]

ok I have an issue and what to see if I can fix myself I have a dell tower OptiPlex 790 I swapped out the hard drive to a 2tb Seagate and upgrade the ram to 8gb

my issue is ever since then my pc shut off out of nowhere sometimes its bad others times it doesn't happen at all nothing screaming hot nor any burn smell I unplugged everything and put it all back together I also ran some performance programs and it runs fine it just reboots by itself

anyone know what it can be PLZ HELP lol don't want to buy another pc

also its running windows 7 I had to download it from online lost my back up cd and dell wont send me another one the keycode belongs to this tower its original

thanks guys

[Awhitewookie]

take out a stick of ram, if it keeps doing it replace that stick and take out another.