Question Need help diagnosing pc issues

[Ihdug]

Hey, I've been desperate these last few weeks. About 2-3 months ago, my pc turned off randomly and wouldn't turn on. Flicking the on/off switch on the PSU seemed to make the fans jump-spin for a second and then stop with no further signals of life. At that point I assumed the motherboard was dead. After ordering a new one (same model), I installed it into my pc and it ran! It ran for about... a day, and the computer shut down randomly again. Only this time, it was possible to turn it on. However, the pc would run shorter and shorter before turning off again. When my new mobo was installed, I did play games and the temperatures of my pc seemed okay, cpu was around 50C and gpu around 60C. The only thing I couldn't check was the motherboard temperatures, as BIOS showed 128C and speedfan showed -48C (Tried touching the motherboard to see if it was too hot while running, but it seemed cool). Frustrated at this point, I thought of other PC parts that have gone bad. First I replaced the PSU and got a new CPU Cooler (As one of the push pins of my old cpu cooler broke and i couldn't mount it anymore). After replacing the PSU the computer ran for around 3 hours before shutting down randomly once more and I was back where I started. Today I went out and got a second CPU to test as I was sure at that point that the CPU was to blame. The computer ran for around 15 minutes with the new CPU installed at normal temperature and shut down again.

My pc specs:
Motherboard: B75A-G41, replaced into same model
CPU: Initially - i5-3330, replaced for testing into a i3-3320
GPU: Nvidia GTX 550 TI
PSU: Was a 550W MARS MP500, replaced into an AeroCool KCAS 600W
Ram sticks: 2x 1600mhz of 4gb each

I've tried everything at this point, even running the pc with only a CPU, PSU and RAM connected. (Had crazy thoughts and even tested out a different wall power outlet, even suspected the ethernet cable and the monitor and even my sound system)
As a last resort, I even took the whole pc out of the PC Case and laid it flat on the ground suspecting a pc case grounding issue but after running for around 15 mins it shut down again.

Any input would be highly appreciated

Edit: Suspected ram too, and tried running the pc with either 1 of the ram sticks installed at a single time and the shut downs still occured.

 

[R_K]

I would suggest check your each RAM with MemTest86, and then if possible check the system with different motherboard other than MSI.

 

[Ihdug]

Ive got a spare ram card and installed it, the problem persisted. Can this be a bios configuration problem of some sort and maybe i need to change something? I have doubts that a new motherboard would fail on my pc just like the last one.

 

[R_K]

Check the PSU, and if you manage to enter Bios check your Vcore or Vid Voltage (cpu core voltage) and Ram mhz and voltage.