I'll wager your sear release lever is worn out.
It's this little thingy:
http://www.brownells.com/handgun-par...prod14527.aspx
With the slide off, it's the inside lever on the righthand side closest to the hammer.
Not hard to replace but it does require some precise fitting and gauge checks to insure correct & safe timing of the decock function.
If you are nearby, I can do it for you. Otherwise, calling S&W for a return ticket is good advice. You're liable to have a hard time finding a local smith that knows how to work on these guns much less time a sear release lever.
Cheers
Bill