Old pc is turning it's self off after 30 seconds to a few minutes

ok i am fixing up an old pc(an old amd 2.4ghz single core 3gb ram) for my neice and it is acting weird,when i turn it on it starts up fine but after a little bit of time(never at the same point)iy will turn itself off it has nothing to do with how much load i have on the pc when this happens a red light(it might be the power light turning red but i am not sure) on the front panel comes and blinks and the only way i can get the pc to start back up is to unplug the power supply from the wall until the light goes off then plug it back in,i installed a new power supply(cause i had to use it's old psu to fix a pc which had it's blow) and has had this problem with both power supplys.

If it helps i noticed after a 3 times of trying to get it to stay running,the north and south bridge heatsinks were hot(not 2 hot 2 touch but still hot).

Thank you guy for the help.

Yeah, sounds like a heat issue. Do you have any way of providing supplemental cooling? Also, might be a dumb question, but are the heatsink fans running? Can you check if maybe the thermal paste is missing/dried?

You could also try booting linux from a USB or CD. That could either rule out or possibly confirm a software issue with the original OS or a hardware issue with the HDD.

try replacing the thermal paste on the proc that usualy is the case with old machines. 

I fixed a friends old computer that way that had the same issue as you.

i do not have a way tp provide any more cooling right now but before the problem even started i moved it from the stock case with 1 80mm fan to an old aftermarket case with 3 80mm fans,also the cpu is not getting hot according to the bios the system temp is 33c and the cpu temp is 42c,also i was in the bios for almost 10 min and nothing happened but when i tried to boot it did that,then i tried to run the startup repair and it still did that.

also after the 10 min the south bridge was hot but i could hold m finger to it all day,but the north bridge was hot enough that i could only hold my finger to it for 3 seconds.

check the psu cables if they are connected properly also if you are touching your board make sure you ground yourself static shock will ruin your comp.

Its not going into hybernate/sleep mode is it?

Sounds like the motherboard is overheating. I would make sure to clean all of the dust out of your case with an air compressor. If you're still getting the same problem after that, then you may have to reapply thermal past to the north and south bridge.

it is not going to sleep also there is little to no dust in it,i will try to reapply the thermal paste as soon as i can find my tube of it.