You said that the throttle and shift cables were replaced. I'd recommend checking the installation and security of the throttle cable at the shift quadrant and at the engine. Pay particular attention to the secure mounting of the moving cable itself, and also the cable housing on both ends. You may have a cable that was not installed correctly and something has come loose.
The throttle quadrant can be accessed by removing the decorative cover and removing or loosening 4 screws. The cover just snaps into place. move it forward or backward and lift. You should be able to remove the screws and lift the quadrant out. The engine throttle cable is on the outboard side of the quadrant and the shift cable is on the forward side. The 2 "nesting" brackets that hold each cable housing include 3 parts:
1. a (maybe) 3 inch long plastic channel that the cable lays in. This channel has a screw that attaches to the quadrant.
2. a "U" shaped flat plastic piece about 1/ 8th inch thick, about 1/2 inch square, with a slot that fits in a groove in the cable housing. This piece fits in a groove inside item #1 and prevents the cable housing from moving inside item #1.
3. a sheetmetal "U" shaped channel that covers the plastic channel (item #1) and holds everything together.
Hope this helps.
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