Carbons can go bad after being shot some, so defintely try several different arrows. Another good possibility is cam timing and/or synch. Also check your axle to axle length and brace height to make sure your rigging hasn't creeped. Along with that, check your tiller. I know squat about dropaway rests and don't want to know anything about 'em. Put a good rest on there, like a GKF Golden Premier, and I could help you get rid of that porpoising.