Amazing service from Rectron.

[sajunky]

I checked and my whole system should use about 380W. It seems the ol' girl can't handle it anymore, bought in 2011.

But works perfectly in my HTPC which barely uses any power.

Check for bulged capacitors in the power supply. It can still work under light load, but over the time it will be worse. High ripple can destroy your equipment. Other cause could be a failing power MOSFET, but it is not easy to test.