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Don`t know your particular control box, and I`m just surmising here. If your mechanic has gone over the box, the cables, and the hookups to the engine and can not find anything wrong, I don`t think the problem is your control box. As far as your throttle creeping on you, there is a simple tensioner adjustment for that. I would bet that when the boat is water tested, he will find some electronic intermittent problem with your motor. Let us know the results, as it may help someone else that's in the same situation as you.
Then again I could be all wrong.

rich