Maybe you should put some shielding around the cardboardbox, just for the fun of it and see what happens.
Can we be sure the plastic box does not have any shielding capabilities? Is there carbon fiber in it?
Another option is to see whether the problem IC880a's are all perpendicular to the antenna or have the antenna directly mounted to it or have a pigtail.
Mine has a pigtail and is laying mostly flat (antenna standing upright of course). => medium problems (~once a month)