When riding a minecart in the nether for a good distance you and the mobs around you eventually start to freeze and move repeatedly I think it's something with chunk rendering. Chunk borders are visible in the distance when mobs and minecart freeze.

Too reproduce make a long track powered at least 300 blocks long in the nether make sure mob spawning is on difficulty easy or hard then send a minecart down the track you don't even have to ride it should be able to see mobs and the minecart start to freeze.