air getting in the cylinder can cause this...
also someone mentioned the velocity fuse (or safety valve), that can cause lifting/lowering problems too, and yes there is only one way to check that, by removing it, if it's located inside the cylinder rod base then you'll have to remove the rod out of that cylinder and take it out and physically inspect it.
one other thing to consider is make sure the mast channels are free of any debris and the tracks the rollers ride on are smooth and free of any dirt or grease. And then finally there may be something wrong with the cylinder itself causing it to drag making it bounce if it is doing this while moving downward.
If this bounce is only happening when you stop lowering then that is air most likely, if the cylinder has a bleeder screw on it you'll have to bleed it out by raising the forks up, cracking that bleeder and allowing it to purge the air, when pure fluid starts coming out then tighten the screw and that should stop the bouncing. (this is only if the cylinder has this bleeder screw, if it does not then refer to the other causes mentioned).