Very very similar problem, tried every thing to isolate, as you are.
Bit the bullet, bought a 1200w power supply, problem gone.
Yes I know, an overkill, but it was so frustrating, I decided to definitely remove the power supply from the list once and for all.
Also noticed how power hungry CPU and GPU’s were getting, that helped me make my mind up to.
Glad it sorted out your case. In general, the most important thing is the PSU quality over absolute wattage beyond a certain point -- I run a 10900K and 3080, both overclocked/overvolted -- which is more or less as much power as you're going to get on a non-SLI system --- on a 650W power supply without issue.
Lower quality power supplies often cannot supply power reliably as they approach their limits. I'd recommend Corsair's RM series, or the Seasonic Focus Plus Gold/Platinum.