First, unless you are a qualified gunsmith... take your Dremmel, and put it as far FAR FAR away from your gun as possible, please!
I'd first check that when you put it back together that you lubed it according to the manual.
Then I'd try those mags again at capacity -1 and see if it happens again. If it doesn't, it could just be that the gun is new and needs to be broken in a bit (load down a round until the magazine breaks in a little)
Just as important as not riding the mag release, is not riding the slide release. It can cause problems as well.
As to WWB being dirty, I've not really noticed too much of a difference, but I clean my guns after every session.