What causes the sander to shake violently?

With age and use, the four rubber posts (that allow the pad to vibrate), can either become too stiff or too soft. If this has occurred, it will be necessary to replace those posts. A twisting of the (4) post can also take place if you or someone has replaced the pad or attempted to tighten any of the (8) screws that hold these post in place. The post are molded rubber and if you look closely you can see two mold marks running vertically up the post. They are 180° apart. The correct procedure is to tighten the (4) four screws that hold the post on the upper side or body of the unit. Then use a pair of needle-nosed pliers to grasp each post (between the sander housing & the pad), and hold the post (so the mold marks do not twist), as you tighten the screw in the bottom-end of that post. Repeat for each remaining post.