Cavitating? Are you sure? Cavitating should be producing one heck of a noise. (You DON'T want to hear cavitating on a 8" products pipeline, for example ...
)
The only thing that could be producing cavitating is if your intake line is blocked, or there is simply too much resistance on your pump intake (input line too small, or too many bends, stuff like that). So you should look to that possibility.
If you're just seeing bubbles, and not too much noise, then more likely the bubbles are venturing in from a minute hole in the joins somewhere. Try taking some PVC cement and sealing up every join and see if that has an effect.
Good luck!