Just because your Power supply has a label on it, saying that its a 1200W supply, means it was a 1200W Power supply when it was NEW.
POWER SUPPLIES DETERIORATE over time
The high value Electrolytic capacitors in them have a limited lifetime, ( often as little as 2 years) before they start to deteriorate more rapidly, and your voltages become unstable under load = CTDs (Guaranteed)
While there are ways to LOAD TEST your supply, to do it accurately requires equipment most do not have.
The TEST most experience is when they start to get random CTDs !!
Just because it looks clean and shiny on the outside, does not necessarily mean all is good inside !!