Power supply issue?

Hello all, recently my PC started experiencing random black screens. Looking at Hinfo, it showed that I was experiencing a failing 3.3v rail while everything else looked good. Looking online, everything pointed to my PSU going bad, so I got a new one. A Corsair RM750e 750 watt. Now, when turning on the PC with the new PSU, the fans spin briefly and go right back off. When plugging in my old PSU, everything still posts as usual, but switching back does the brief fan spin then off. Looking online again, I thought it might be a bad PSU, which looking back seems stupid, but now I have a new PSU, and the same issue persists yet my old PSU still posts as normal. Any thoughts on what my next steps should be?