This, and everything else too. We really need to know not just the derailer and shifter, but what freewheel and chain you're running with the combos so far. Some of the 5-speed chains from back in the day combined with unrefined Atom freewheels (or Schwinn-licensed Shimano copies of Atoms) could be quite the noisy mess to shift.

Pictures would help too, for if there happens to be a recurring setup issue, it may be visible.

Question for the OP: Generally, in your experience, what combinations have you had that you've been pleased with? It'd help to give us a point of reference.

-Kurt