Computer Freezing at random times

[Jstagzsr]

so i bought a new used computer from a computer store, he says he runs plenty of diagnostics and tests on everything he sells and ive bought stuff off him in the past and i had 100% good experiences and always got great deals on great stuff there before.. so when he told me it worked, i believed him.. but now that i left the store, and since "all sales are final" im stuck with this computer.. which isnt necesarily a horrible tihng, but still very irritating...

 

it is an "old" server tower.. it has a quad core xeon e5320 in it with 16gb ecc ddr2 5300, a 1gb graphics card, a ssd, and all the regular normal stuff... coming from my laptop to game on which wasnt even half as powerful, this thing has been amazing. running most games on high graphics settings is always awesome.. i ordered another 16gb ram and i ordered dual matched xeon e5450 processors off ebay.. so ill have 8 cores at 3ghz with 32gb ram here in a few days..  but the issue is making me nervous.. is it a bad motherboard? bad memory? there IIISSSS a bad SOMETHING in this computer and i have no idea what. ive ran my own diagnostics on every single thing i possibly could.. all hardware and windows.. it has the latest updates, everything seems to be working perfrectly fine...

 

but then randomly it will freeze up and the only thing i can do is hold down the power button until it restarts.. ive had one other computer in my life that did this, froze up randomly. but THAT computer, when it froze the sound would also freeze and give me a loud repeating sound of whatever the last sound that was playing was.. hard to explain.. THIS computer when it freezes the sound completely cuts off and the screen freezes.. no blue screen of death, no orange screen of death, nothing... but diagnostics says windows is fine (i reinstalled windows and it worked perfectly for 3 days), but then this morning it did it again.. so obviously its not windows.. the memory test brings up no issues.. so then whats left it the motherboard and the cpu.. if its the cpu i dont really care because im replacing it in a few days anyway, but if its the motherboard then im doing nothing but putting all this money into a machine that will always be broke until i get a new motherboard... but if i get a new motherboard im NOT going to go with a server mobo to run high end old parts, im gonna rebuild a whole new computer. which i cant afford, hence the 10 year old "gaming/server" computer im running.. I also do a lot of video editing and music production so this computer WOULD have been perfect...

 

does anyone have any idea whats going on here????? PLEASE HELP!!!!!!!!!!!!!!!!!!!