cruise on and engine brake off driving me crazy

[Chamberpains]

This sounds very similar to a post hongo88 had of a 2012 Cascadia about a month ago. Unfortunately he never updated what he found or what he ultimately he did with the truck.
I haven't figured out how to post links from this site yet. But it's labeled under General discussion, 2012 Freightliner Cascadia help needed. You may want to give it a quick read to see how we approached tackling that problem.

It sounds like you have a cruise control switch sticking or flickering. But I'm only taking a shot in the dark.