How old is "very old". More than 10 years, is really pushing it, even for a "good quality" PSU. Their capacity for power output degrades over time, and the Ryzen R9 3900X does take a decent amount of power.
I KNOW PSU prices SUCK right now, but... you might have to suck up, and buy another one.
You could probably try your old one, though. Maybe some of the other old-timers will comment.
My personal philosophy on PSUs and HDDs, is to mostly, pro-actively replace them after so many years (or the warranty runs out). I haven't done that for the HDDs in my NAS units, because they're running in RAID-5, and were shucked, so there's not really any warranty to worry about. But if they slow way down, and act funny, then I'll replace them regardless. In a client PC, I replace HDDs every 5 years or so, and PSUs every 3-5 years, depending on quality, or when the warranty runs out, whichever is longer. (Some PSUs are really high-quality, and are warranted for 10-12 years of operation.)