Is my MOBO a PSU killer or am I just unlucky?

[Gurbo]

A while ago I started having some severe problems with my pc, which after some research and tinkering I traced back to a severely undervolted 3.3V rail (slightly over 10% lower, with the other rails also starting to undervolt a bit) according to the BIOS.
I switched PSUs for a spare I had lying around which worked perfectly fine at the time, and 4 months later I find myself in a similar situation. The 3.3 rail is starting to undervolt, the other 2 are fine, but the issues are coming back.

Additional info: It's summer here, the problem started again in the first really hot days, and if I add beastly amounts of cooling, it somehow holds up. Three extra pc fans don't do the trick, but opening the case and fixing a 30 cm radius one blowing into it does the trick. Underclocking and undervolting don't help at all.
The PSU is not being pushed hard in any way, as an exaggerated estimate I can't see it going over 40% load, 30% might be closer.

From this, I see two possible scenarios:
-My Motherboard is killing the PSUs I pair it with.
-I'm either really unlucky with my PSUs or my lack of knowledge (and money) at the time made me pick extremely unreliable ones.

What am I missing?

I've kept this rig going for way too long, and I would really appreciate your help if you give me a hint on how I can keep her running into a very deserved retirement instead of just dying at the side of the road. It's the first one I built on my own, and after almost 8 years, I kind of grew attached to it.

EDIT:
Specs:
CPU: Athlon 64 x2 5600+ (Windsor, 90nm)
MOBO: Asus M2N-VM DVI
Current PSU: Codegen 300x 500w
First failed PSU: Shark Net SN-PS 60 600w
Memory: 2GB DDR2 Kingston, unknown model.
GPU: GeForce 8400GS Rev.2 (Can't find the manufacturer, it was a hand-me-down after the original GPU failed)
Hard Drive: Samsung SpinPoint T Series HD501LJ 500GB

What really puzzles me is the fact that both PSUs are failin in the same way, and generating the exact same problem.

 

[ko888]

What are the complete specs of your system?

 

[Dark Lord of Tech]

What PSU's are you buying.

 

[Onus]

You will need to provide brand and model (not just wattage) of the PSUs, and also your motherboard, and the CPU you are using.

 

[Gurbo]

CPU: Athlon 64 x2 5600+ (Windsor, 90nm)
MOBO: Asus M2N-VM DVI
Current PSU: Codegen 300x 500w

Other components:
Memory: 2GB DDR2 Kingston, unknown model.
GPU: GeForce 8400GS Rev.2
Hard Drive: Samsung SpinPoint T Series HD501LJ 500GB

Currently looking for the first failed PSU for the model, I just remeber it was 80+
Got it: Shark Net SN-PS 60 600w

And SR-71, I bought whatever I could afford... sadly, the prices for computer parts here start at x2.5 US prices up to 5-6 times. A few months ago, an EVGA 600B was about US$170.

 

[Dark Lord of Tech]

Codegen 300x 500w is a very poor power supply.

 

[Onus]

Codegen sells PSU-shaped objects that are unfit for purpose.
If you'd like to keep this machine functional, one of the 300W 80+ bronze Seasonics would probably do...BUT...
It is possible that your motherboard is so old it may expect a -5V rail in order to function properly. Unfortunately, no modern PSU will provide that, so you will be forced to choose the lesser of many evils among other PSU-shaped objects that still offer it.
If your current PSU-shaped object does not have a -5V rail (if not clear on the label, it is a white wire in the 20/24 pin motherboard power cable), you'll be able to select something decent.

 

[Gurbo]

Now I know... at the time it was a mix of lack of knowledge, not having much better options with the money I had and desperation after a heatwave fried the first one I had bought.

 

[Gurbo]

I was already thinking about buying something in the 550-650w 80+ gold or up from a decent brand for my next rig first and use it on this one until I can hunt down all the other parts, but as long as I don't know if the faulty PSUs are the problem or the symptom, I'm a bit hesitant to spend on it.
Is there any way to be sure the fault is on the bad quality PSUs or if something else is causing them to fail?

 

[Onus]

About the only way parts will damage a PSU is by shorting or otherwise overloading them. Modern, quality PSUs have protection circuits against this problem, so I wouldn't worry about it (cheap PSU-shaped objects are often missing protection circuits; hey those cost money!)

 

[Gurbo]

Thanks, having both of them die in the exact same way was making me a bit nervous about buying yet another one. I'll do that. I experimented a bit more today, as it was colder, and it seems like it was just a heat problem.