My next move would be a “slot swap” to isolate the issue to either the board or the slot and its connectors and cables.
i.e. Move the problem board to the first slot and vice versa
Leave any cables with the slot.
See if the problem travels with the board or stays with the slot and its connectors and cables.
I would also look closely at slots to look for slightly bent metal, chipped plastic or possible small pieces of dirt or packing material, jammed in, etc
Just thinking out loud.