I have some consistent behavior to report now. It seems anything that initiates drum rotation results in the same problem. The drum moves over from rest position about 6" and begins spinning. It then spins for about a minute, and stops.
If it was initiated with silverfast, say for a prescan, it exhibits this behavior during "check lumens", then reports a 702 error on the scanner then stops, and Silverfast reports "cannot focus". I doubt that's the problem, I just think the software is waiting for the scanner to complete calibration and focus to move on. If I hit OK in the alert, it resets the scanner to "ready" and I can start another prescan, same result.
With the FST diagnostic, it does fine with the initial tests that do not initiate spin. But the rest, the same things happen- drum scoots over about 6" and then spins for about a minute. The difference with the FST is that when the rotation stops, it immediately starts again, for another minute, and we're stuck in this cycle. Not a loop test, but a one time test.
Any suggestions at all would be helpful, I'm very close to calling time of death.
Thanks, Tyler

previous troubleshooting and activity are here-
and here-
-however I think the above behavior is symptomatic of the main problem, yet undetermined