The word "jerks up down" can be tricky to find synonyms for, but here are a few options: jolts, bounces, rattles, shakes, wobbles, quakes, oscillates, vibrates, shimmies, and convulses. Each of these words suggests a quick and sudden motion, often repetitive in nature. Whether you're describing a bumpy car ride or the shaking of a washing machine, these synonyms can help you add variety and interest to your writing. Additionally, they can be used in figurative language to imply a sense of instability or uncertainty, as in "the stock market jolted and bounced unpredictably".