Watching the video, you seem to do alot of riding in a gear higher and slowly lugging the motor...not something a big bore mxer is designed for.Sounds like it's just loading up and needs to be cleared out.
If they are lugged to long the crank get full of unused fuel.
It;s a simple case of starting the bike and holding it wide open, eventualy it will rev out nice and clean. This can take up to a minute.
Does this happen when you are riding in wide open territory or just single trails?
I had exactly the same thing happen to my sons KX 60 when he was first learning to ride a geared/clutch bike.
Have you checked your reeds for wear.
or
You need to replace your clutch side hand grip