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

Apr 24, 2019
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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