First, put on some reading glassed or grab a magnifying glass and look at each stop notch on the cylinder. It's possible that scrubbing out the stop notches with a toothbrush and some solvent may cure your problem. It's been my experience that the cylinder stop notches can be a magnet for crud, which can prevent the cylinder stop from fully seating. BTW, I just love it when something this simple fixes a problem for me, so out of habit I ALWAYS start with a good cleaning.
From your description is sounds as if you have a carryup issue. The solution to this is to fit a slightly thicker hand so that the gun will properly carry the cylinder into full lock. At the same time I would suggest that you have the spring for the cylinder stop replaced.
That new hand will have to be fitted to your gun, which is a somewhat finicky task that does require some precision measuring instruments. It's also something that requires a fairly high level of skill, so I would advise that you find a competant gunsmith to tune it up.
Note, carryup problems such as you have described can sometimes occur in a gun with a near perfect state of tune. If you have been "staging" your trigger in double action, you may find that pulling the trigger in one continuous stroke solves your problem. Doing that allows the inertia of the cylinder to carry the gun into lock even if the carryup is just a tick short.
S&W intentionally builds their revolvers on the loose side for carryup to reduce hand fitting and insure that it will function properly in a Combat mode of shooting even when it's not been properly cleaned or maintained. They also advise AGAINST staging the trigger in double action shooting in their current manuals. If you are in the habit of staging your trigger in double action, I would suggest you break that habit.
If you don't stage your trigger in double action, then without a doubt you need to have your model 57 tuned up. While it's not cheap, due to shipping costs, I personally would have S&W do the work. Turnaround on service work seems to run 2-4 months from the posts I've seen concerning this but the results have been 100% positive in every post I've seen. They'll not only correct the carryup, but will also make sure that gun is 100% perfect in every respect before returning it to you. The only complaint I've seen in regards to S&W's service is that on a few occasions they didn't return the gun with the grips it was sent in with. Tip, remove your grips before you send it in for service or send it in with a cheap set of Pachmayr's.
