Do you have more than one magazine, and if so, does the problem occur with only one magazine?
I had a similar problem, albeit with a different gun. First it was one magazine, then it started happening with the other. One mag was after-market, but it was about the same age as the gun and original mag. I found some wear on the followers (refer to cane's picture). I replaced them both, and the problem disappeared.
As others have said, check your grip to make sure that you aren't contacting the slide stop. I'm sure you aren't when you take your grip initially, but sometimes our fingers slip during shooting, especially when the weather's hot and our hands get sweaty.
And if you think it might be the reloads, run some factory ammo through it and see if the problem persists.
"An armed society is a polite society." Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
William S. Burroughs