My neighbors mower does pretty much the same thing twice a year, I remove the tin shiftier cover and squirt ATF back into the variable pulley and tap the center part that is intended to move up and down with the peddle position with a wood dowel from top and bottom and it comes free and it works fine again. Looks like when it is stuck down and the peddle is let up the front belt is loose enough to slip off.Been squirting it twice a year for three years and it keeps going. He never mentions it slipping from the ATF and it does not make the belt come apart.I think it gets sticky or gummy from mowing wet grass, looks like a car wash coming across the yard. It says Shift on the go MTD on it.I keep hoping it will blow up before a major repair, Vise grips on the fuel line for a shut off and o ring on throttle shaft because it sucks air around it, every switch is bypassed,uses a screw driver on solenoid to start, Air up the tires and top off the oil and put the wet side of the couch cushion down. Good old mowers if they would have stayed away from the plastic parts.