I have a ride on mower with a briggs and stratton engine which starts and runs no problem, when you drive it every minute or so it seems to jerk and notice it more on a small incline. Have cleaned the card and even changed it for another off a mower but still does the same ?
Worn drive belt, worn idler, stuck Idler arm loose transmission pulley worn diff partially broken drive key ( where fitted ) sticking brakes stretched clutch spring or spring torn a groove in the anchor hole .
All of these can cause your problem where the actual wheels get ahead of the drive from the engine so the belt goes slack for a second then catches up
