[tsubaki]

I would be inclined to think it was both the control box and cables causing the problem.
If you got it on the list to replace, do it and see how much it improves it.
The center post (in the control box) that the shift arm rides on is always subject to corrosion, causing the nylon bushing to swell and cause friction.
While you're in there, remove it and hone out the hole that the bushing goes into and clean and grease everything else that moves with waterproof grease.