This one can be tough to nail down. Basically you have one (or possibly more than one) card that is so fubar, that it has the backplane so jacked up the SLC can't even tell where the garbage originates.
Get your spare cards for each type in use and swap them one at a time until the symptoms change.
Another potential problem is ground current noise but most likely there will be a faulty card or a serious load problem on a relay card.
Relay cards should be your first suspects, and even without spares you can remove the wiring arms from them and see if the problem clears. 1746-OW16 cards have a couple of relays that are near enough to the backplane that a failed or failing inductive load connected to one of those relays can induce a current onto the backplane that will screw things up for the whole rack.
Another thing to try is the rack power supply. If you have a new spare power supply, it is easy enough to change that out too.
