A severely compromised PSU that hasn't outright failed yet may limp on for a long time if kept under its broken limit. Try a new PSU. If the PSU is in trouble, it may happen that the next time you try to boot it will have the same effect on the older card, or it may take 6 months before outright failure.
When that happens, if it isn't a decent quality unit, it may take a lot of your system with it. That would be reason alone for myself to prioritize crossing the PSU off the list of possible sources of the issue.