I just removed my riser and mounted my GPU stock standard and it seems to be running a lot better.
I can run a benchmark on 3Dmark without it crashing or an error popping up.
Check your mounting - try the second PCIE slot if you can, see if there is any difference.
I am getting the same issue - I have a brand new 1000W PSU, just installed a new B550a-pro and running a 3090.
PC will just randomly hitch up and it might be ok, or it does a full restart.
I thought it was the PCIe gpu slot on my old Mobo (B550F GAMING WIFI) that was busted, as the problem went...