Problem: slide fails to go into battery completely

History facts: My S&W Model 108282 was manufactured early in 2010-JAN date of factory test shot. It was originally bought by a fellow that did not shoot it {ever}. He did, however take very good care of it; he had it about 18 months and had field stripped it a time or two to keep it oiled / maintained. I bought the gun from him a couple of months ago. I bought four new Wilson Combat 47D magazines (it came with two factory magazines but I didn't use these).

I loaded the four Wilson magazines with PMC Bronze, 230-gr, FMJ and went to the range. I had the same problem with rounds from each magazine (so I'm pretty sure it's not the Wilsons). I would rack the first round and the gun would shoot fine for the first three or four rounds and then out of the clear blue, the slide would need a little [manual] help from me to close completely. {Very fustrating} After that, it would fire fine again for several more rounds - and then repeat the same problem.

I didn't have any problem with ejections and also no problem with loading. Each time it happened, there would be a round completely loaded and ready to fire...it's just that the slide didn't close all the way.

I field stripped the gun to clean it after shooting and everything looks good and normal to me (with my un-gunsmith trained eyes). I cleaned it throughly and put a little {very little} Tetra Gun Grease on the slide.

Does anyone have any idea what's going on with my beautiful 108282? I want it to work as good as it looks!