I have the same issues !! I have a couple of raid drives and third 10T - a second dell monitor connected to my iMac pro 18 core....If I loop a time line in fcpx, loop audio in Logic, run the Black Magic disk test my system will freeze, still playing audio then give up the cursor then hang fully and crash = in the spin dump we get the watchdog time out !!!! I am on 10.5.5
looks like its some kind of GPU issue