Who is guilty? PSU? Motherboard? or both?

[i-am-L]

This is my rig:
CPU: AMD FX4100 at 4GHz
Board: Gigabyte 78LMT-S2PT (Rev 4.0) 3+1 phase
PSU: 550W (cheap local brand)
CPUCooler: CM TX3 Evo (attached additional fan to it)
System fan: 4300rpm fan
HDD 1: 500GB Seagate Barracuda
HDD 2: 160GB WD Caviar Blue
GPU: Nvidia Geforce GTX 750Ti 2GB (Zotac, doesn't require ext. power)
OS : Windows 10 Pro x64

Motherboard had a problem a year back. When Power button was pressed, the cpu fan, system fan and case LED would start but the system won't start. No beep sound from buzzer and no display. [I didn't have a graphics card at that time, m'board has onboard gpu]
Service center repaired the board.

Now I have a really weird problem. The PC turns off randomly sometimes reboots. I don't know whether it's a PSU problem or is there something wrong with the power phase of m'board.

Note: some events that occurred would really sound like I am just kidding or lying but no, they actually happened today.

Log of events that occurred today:

•• PC turned off while entering the password for logging into windows. Power button on chassis didn't work after that.

Note: after PC went off, the num-lock LED was lit and could be toggled on/off via numlock key, same goes for capslock and scroll lock. Then I switched off the cpu power from the spike-guard and when I turned it on again after about 5-6 seconds, the numlock key lit again and could be toggled on/off.

The PC started again when I tapped the PSU with my finger ( the chassis did shake a little)

•• PC went off while surfing the web. This time I just left the PC as it is, I didn't even tap the PSU.
I left the room to take a pee.
When I returned, the PC was on and was at windows logon screen. [ I was alone in the house]

•• PC went off while watching an episode of VGHS on youtube.
Tapped the PSU again which shaked the chassis also a bit and the PC started again.

•• I was playing CS:GO, not even 5 minutes had passed after starting the game and entering the server. The PC went off. Tapped the PSU and PC is again up and running.

I am really fed up of this. It isn't a software problem for sure.
Now I don't know what's the problem. Is PSU at fault or is it the motherboard?
I am doubting on the motherboard also because of the numlock led that can be toggled on/off after the sudden turn off and the power on that took place automatically when I went to pee,
Also the motherboard had some issue a year back.

Please help me, I have received cooldowns in CS;GO due to this. I am working on a WPF and C# project, I have also lost my unsaved code I typed due to this problem.

 

[Alain Auclair]

don't tap the case bad for the hdd.
chnage the power suppply and/or motherboard.

 

[lunyone]

Well since you probably have a subpar PSU, I think you should upgrade your PSU to a more reliable one (Antec, Corsair, Seasonic, PCP&C, & XFX are the brands I look at). Also make sure you get at least a 400-500w PSU from the previously listed brands. Make sure you get at least an 80 PLUS Bronze rated PSU, because they are more efficient and you usually get better power regulation with an 80 PLUS Bronze PSU than one that isn't rated at all.

P.S. If you have a friend with a different PSU (hopefully better built) you could try theirs out first, to verify it is the PSU.

 

[ksells]

I'd also say its a PSU problem