TomGeorge, I didn't mean to hijack your response, but was wondering, after you mentioned it, then confirming after he mentioned they were "eltako teleruptoren" which are zero crossing, which he's using with a magnetic ballasts, may create an illusive problem like he's experiencing? I've seen such a problem in other scenarios, but not one exactly like this, since the relays driving the contactors would seem to completely isolate them from the shift registers. It seems as though if it were related to the contactors, that it would only affect the particular light and not create the cross-talk he's experiencing.
Will look forward to, as I'm sure Orion will, your response to the additional information he's provided.