If it starts and stops I presume you mean the motor is pulsing? I recommend giving the filter a really good wash, the HEPA part can get clogged even with water the pores are that microscopic, so ideally it's best if you put it in the washing machine with a little bit of fairy liquid
When the motor is running it'll struggle to breathe so the motor will pulsate due to a clogged filter, it's a very common issue because people think their filters cosmetically look clean when they still need washing thoroughly