Driving down the highway, Cruise control will only sometimes engage in 5th gear, downshift into fourth maintain speed cruise will work every time. Hooked the Snap On scanner up to monitor the cruise module, in 5th gear it flashes yes/no rapidly under cruise enabled and constant no under cruise engaged. Downshift into fourth the cruise enabled stays a constant yes. Then hit the set button cruise engaged says yes, and it maintains speed. What could be causing this problem?