Old PSUs were easy. They tended to either work completely or not at all. Now they can keep on ticking, but may not produce the amount of power for which they are rated.
Best guess from here is that your PSU no longer is producing enough juice to power everything in your rig. Whether it was only barely able to keep up before, or has degraded considerably isn't really important.
It's easy enough to check if it is the PSU by swapping it for a spare, but most people don't have an extra on hand. If you don't have a spare, may as well get one that can support your current rig, as well as one you might build in the future. I always like to get one that has more capacity than I think I will ever need, though you don't need to go crazy here.
If it isn't the PSU, then something in your rig is requiring more power than it should, you replaced a faulty power switch with another dud, or something more complicated.