I would pull on the fd cable by hand and see how far it throws the chain, if it now throws the chain off the big ring it's telling you that your cable wasn't tight enough ( and you'll need to screw your limit screw back in). If you cannot get the fd to do this (assuming its aligned correctly) then you most prob have a campatability issue. I would bring it to lbs anyhoo, fds can do your head in if you don't deal with them daily.