Question Automatic shutoff after 2 min unless sitting in BIOS

[LimeyTie9603]

Background:
I was given an old PC. First thing - blew out the dust and dog hair and lint balls.
Powered it on the next day, and went through the BIOS just checking it out. No problems encountered.
Problem Encountered:
No HDD, so I rebooted and attempted to boot to Linux from a USB stick. And within 2 minutes the machine powers off - just like if you flipped the switch on your surge protector. This happens repeatedly.
Troubleshooting Attempts:
At first, I thought it might be an overheating CPU -even though the fans are all working. I can immediately boot back up and go back into the BIOS and check on the hardware monitor and see that my temps are not elevated. (less than a minute elapsed from auto shutdown to back into BIOS)
It seems as soon as any real work hits, like while or when it finally boots into Linux, it just promptly shuts off. I even put in a HDD and tried to install Windows, it started me through the menu screens, but again, it auto shutoff after ~2 min.
I pulled the dedicated graphics card just in case, and I even tried with one stick of ram at a time.
I also tried using a different PSU that is working in another case. Same results.
I finally used my USB drive to boot into Memtest86 and it's ran for 3+ hours and completed 3 full passes with no errors.
I also flipped the Easy OC switches to "Off" and reset the CMOS so it cleared the BIOS back to factory default.
Specs:

• AMD Athlon II X4 640 Processor
• 2 x 2GB DDR3-1280 RAM Crucial Ballistix Sport
• MSI NF750-G55 motherboard
• Thermaltake TR2-500W PSU

FWIW: Running memtest86 for 3+hours reported a stable CPU temp of 24 deg C.
Sitting idle in the BIOS for at least 15 minutes I recall seeing CPU temps of 33 deg C. (87 deg F)

 

[Grobe]

Well - the motherboard is like 12 years old, so it have lasted for very long time. If the computer have being used regularly then this is expected by now.

 

[LimeyTie9603]

Well - the motherboard is like 12 years old, so it have lasted for very long time. If the computer have being used regularly then this is expected by now.

I would expect it to not turn on, or to be unstable. However, I've never encountered a system to run for hours in the BIOS or in Memtest86 but unable to finish loading an OS.
My question is what part of loading an OS could be causing it to trip a shutoff event?

 

[Grobe]

Well - just maybe - this point to the PSU. As OS loading, it may activate more of the equipment and for that reason:

• More power used. If this make the voltage unstable isn't known, but cannot be disregarded either.
• Some component other than the motherbard may be very sensitive to voltage not being stable.

So if the PSU is to blame, then one or both of the above could potential be true.

 

[LimeyTie9603]

Well - just maybe - this point to the PSU. As OS loading, it may activate more of the equipment and for that reason:

• More power used. If this make the voltage unstable isn't known, but cannot be disregarded either.
• Some component other than the motherbard may be very sensitive to voltage not being stable.

So if the PSU is to blame, then one or both of the above could potential be true.

I agree, the PSU is the most logical next step. However, I did install an alternate, working PSU, and the behavior remains.

 

[LimeyTie9603]

Is there something else I can try to continue troubleshooting?

 

[Grobe]

Is there something else I can try to continue troubleshooting?

Unfortunately the motherboard itself tend to be the failed part if the PSU prove good.

You can of course have a wild guess and removing one ram stick just to see where that leaves, it doesn't hurt after all.

 

[LimeyTie9603]

Unfortunately the motherboard itself tend to be the failed part if the PSU prove good.

You can of course have a wild guess and removing one ram stick just to see where that leaves, it doesn't hurt after all.

Yeah, you may have glossed over that part, but yes, I tried with one stick of RAM at a time, and still had the issue. I still wasn't convinced, so I ran Memtest for over 3+ hours (on this problem motherboard) and the RAM had no issues.
That's why I decided to ask the internets - it will run Memtest for hours, but try attempting to load/install an OS and it auto-shuts down.