1) According to the official specs, that PSU isn’t enough for that graphics card (PowerColor suggests a 900W PSU). That aside, a 750W PSU with enough miles on the clock to constitute say half its life will have dropped in capability (which is normal for a PSU) so it probably can’t serve up 750W any more anyway. Another thing to bear in mind is that modern GPUs recently (around 2020-2023?) went through a phase of drawing a tonne of extra power for an instant which was causing seemingly sufficient (according to the figures) PSUs to fall over and crash the system. While I think that problem has been brought to heel since (because it was getting out of hand), it wouldn’t surprise me if these micro spikes still occur even on a lesser scale. Furthermore, I bought a 750W PSU for my 2023 X3D system because I changed to a PSU for a GPU that had a minimum recommendation of a 650W PSU (which I already had), yet ‘only’ had 1x8pin and 1x6pin power needs, compared to your card’s 3x8pin.
2) You’re using a top vent as an air intake source? Heat rises. Top vent should always be to exhaust air. You need to encourage an air flow through the case, e.g. from front out the back, the hot air that escapes via the top vent can handle itself. Front fans should pull air in, and rear fans (if any) should be sucking air out.
3) Try a CPU-only stress test (e.g. Prime95) for half an hour and see how it fares, at least to narrow the list of suspects.
My bet is that the PSU isn’t beefy enough any more.