With your test shorting the green wire, you've effectively narrowed down the failing component to the PSU. Good job, not everyone knows how to do that.
I don't have any knowledge about that model, but if this is your 2nd that died in under 3 years, then move to another brand or model.
PSUs produce less power as they age. If your system needs 400W and you have a 450W PSU, then you only have a 50W difference between full power and potential damage to your components by under powering them. Yes, this is a thing.
Generally, I don't put less than a 650W PSU in even a basic computer. With your system needing 400W, this gives you plenty of headroom for the PSU to slowly fail over time, as well as room to grow (CPU, GPU, or RAM upgrades, more HDs, etc.).
Electronics only pull as much power as they need, so your system will only pull 400W. You don't have to worry about the PSU "pushing" 650w (or whatever you choose), since it doesn't work like that.
Also, a PSU tends to be more efficient the farther away you are from it's max wattage. This happens because you are putting less stress on the components. Putting less stress on the PSU also helps is age more gracefully.
My recommendation is to get a 650W 80 PLUS Bronze or higher wattage PSU. The difference between the 80 PLUS levels is minimal, but I figure Bronze level uses better parts.
https://en.wikipedia.org/wiki/80_Plus
EDIT: If you have the same issue with the new PSU, look into getting a power conditioning UPS. There may be something with your incoming power that is frying your PSUs.