This is a problem that occurs when a combonation of three things may happen:

1) The level is too big to hold in Jazz's graphics "sandbox."
2) The level features too many animations.
3) The level uses a tileset that is quite large.

Usually, when this occurs, the player will disappear because the game is limiting animations and taking away from the player's animations. If this becomes bad enough, certain actions (such as Spaz's sidekick) may cause crashes. At the worst, the game will simply crash when moving. To fix:

1) Reduce animations.
2) Reduce unneeded level size.
3) If all else fails, restart with a different tileset.

Good luck.