Whirlpool Dryer Drum Not Turning

2019-9-3my sister in law has a whirlpool awz7303 condensor tumble dryer thats a few years old nowore often than not, when you press the start button, you can hear the motor trying to start the drum turning but it doesnt manageround every 5th attempt, it fires into life and then works fine, turning in both directions and drying the clothes.