Why is the saw blade bouncing up and down?

Tension is too loose, the teeth are dull, or the material is too thick.

A blade that does not have enough tension will wander. It will also flex from side to side, making for irregular or angled cuts. If you press too hard on a loose blade, it will usually snap.

A blade that has too much tension is more susceptible to breaking and tends to pull out of the blade holders. In general, it is better to make the blade too tight rather than too loose.

If this answer is acceptable please remember to return and mark it.