Question PC Radomly Shuts Down, No Display sometimes and ram doesnt read correctly?

Apr 24, 2019
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Hi all first time posting, hoping anyone can help me because I'm out of ideas. I built a PC around 5 years ago now (old I know), and a few months ago it started to shut down randomly (no shut down screen just off). Sometimes it would be fine for a full day and not have any problems then the next day I would go to turn it on and it would either, not turn on at all/fans and lights would come of for a few seconds but then turn off/come on but no display on the monitor/boot up fine for a few hours then turn off and not come back on/turn on and work fine. Also I have 12gb of ram but it usually only says I have 4gb or 8gb installed. I bought a new graphics card to see if that was the problem which it wasn't, now i'm unsure if its possibly the mobo or psu?
OS: Windows 10 64 bit
CPU: AMD FX - 8150 Eight-Core Processor -3.6GHz
GPU: AMD Radeon R9 270x 2GB
Ram: 12GB (3 x 4GB) DDR3 Corsair Vengeance
PSU: Cronus Modular 750w Model E-CR750M
Mobo: Gigabyte ga-78lmt-usb3


(A tricky situation to solve I know but any help would be greatly appreciated)
 
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