I had a Quad 400, the happiest day I had with that thing was when the limb cracked and I could justify buying another crossbow.
If all else fails, try tightening EVERY screw and nut on the crossbow (including scope mount & rings), and try to LocTite them all. I think there is just too much recoil on these bows, mine was never consistent, but keeping all the screws tight helped.