DIAGNOSIS Excessive vibration is almost always caused by a worn or damaged "counterweight" or wobble plate mechanism inside the gear housing, or by using a severely bent or poor-quality blade.
FIX Start with the simplest fix. 1) Remove the blade and install a new, high-quality, straight blade. A bent blade will throw the whole balance off. 2) Make sure the blade is inserted all the way and clamped securely. A blade that's not fully seated will vibrate horribly. If a new blade doesn't solve it, the internal mechanics are likely worn. The wobble plate and connecting rod can develop slop.
IF THAT DIDN'T WORK If the vibration persists with a new blade, the internal counterbalance system is faulty. Opening up the gear housing (remove the three screws on the black gear case) will let you inspect the wobble plate and rod. Look for cracked plastic or excessive play where the rod connects. Replacing the gear housing assembly is the permanent fix.
HEADS UP Never use the saw without the shoe properly adjusted and resting against your workpiece. Letting it run freely in the air maximizes vibration. Let the shoe absorb and transfer the vibration into the material you're cutting.