Do you have te book? This is the asme 2000 version since it has redundancy, but is it the normal f1-f8version or the newest version with more dips?
I'd swap ribbon cables next. I've had some odd issues in the past from ribbon cables that never move but connection got iffy (especially ecospaces). After swapping ribbon cables I'd run it to see if problem went away or you've ruled that out then move on to checking power at points on board and watching cpu inputs. See if there's any triacs on that circuit on the i/o board too.
