Hi and thank you for the quick reply.
ASRock Industrial tech support said the same thing about the DRAM requirements, then told me to contact my distributor when they had no solution to the test failures. Each of two 32GB sticks works successfully by themselves in both slots, and the two sticks together fail consistently regardless of the slot. This was all done using the default BIOS setting for fan speed (automatic fan speed control). Changing that setting to maximum fan speed results in lower temperatures and the RAM generally works (just one failure). I'm retesting this case now to see if it works for an extended period. Kingston tech support has already replaced the original DRAM and the replacement fails in the same way.
So, now I'm asking for someone who HAS successfully made one of these systems work to find out what RAM they used.