the engine will run awesome then with no warning will run like it has a intake restriction. checked vacumn slides for cracks or pin hole(good) no intake restriction, removed air filter to watch carb slides, they will not slide into carb body. it acts like the timing will not advance. i diconnected the neg. grnd. hooked it back up and ran great for a few hours then went into the same condition again. possible igniter box or crank sensor , no codes are stored . any ideas , all dealers are stumped.