Jake brake/ cruise control works intermittently

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I have a 96 Peterbilt with a series 60 The Jake brake as well as the cruise control only works sometimes. And if it is working it will stop if you push in the clutch or are in cruise and start up a hill and put a strain on the engine. I had discovered that when it stops working I could turn the ignition off and then back on and it would start working again. But after so many times of doing that it has quit all together.I have spent over 800.00 with mechanics and nothing has worked I Have replaced the throttle positioning sensor but dont know where to go from here.