Unlucky 9800x3d?

Trying stabilty on Gigabyte X870 with PBO -13 all cores. CPU clocks average 5Ghz, but getting 2 core failures on Prime Blend stress test within 5 minutes (!), with max temps across cores 56-61C. Using an AIO 360 cooler.

Have I got a poor performer or can I do anything to improve this other than per core PBO? Could it be a thermal paste / mount issue?