Fanuc 18i controller 926 FSSB alarm

We have been dealing with an issue on a machine I was wanting to ask you guys about. It is a 926 FSSB alarm, and the drive will have a "L" fault on it. I have changed the axis card, servo drive, servo motors, and wiring, swapted the fiber optic cables, and the issue will always stay with the drive, we have found that if you are to leave the electrical panels open then this fault will not occur. I have cleaned the cabinet cooler, and it will still fault, it will just take longer.

Do you have any thoughts, or any questions?

The L can indicate a defective fiber optic cable between the drive displaying the L and the next one in line. If this is the case the next drive should be displaying a U. Is this the case?

Page 644 of GEFanuc manual 63005en shows how to isolate the problem to a particular component.