RE: Really Bad Bounce On The Bench
First off that's a good way to cut the crap out of your arm. You should use something heavy through the skids (not your arm, lol) or actually screw it down to the bench.
Now that the safety lesson is over,,, check your servos. Be sure that one (or more) dosen't have a stripped gear. It's pretty common to strip those plastic gears and that would cause it to jump or bounce as you described.
Good Luck.