Have you tried setting the 850 to RF only for the cable box?
If there is ANY chance that the box is receiving the 250's IR (slightly delayed) as well as the 850's IR, it won't function properly. Keep in mind the 850 has a very powerful IR blaster built in to it. I've noticed that you can control things in a room when the remote is aimed in the opposite direction.
Is the cabinet completely sealed when the doors are shut?
On the 250, are you using emitters our the built in blaster?
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