I'd second what Kowalski says entirely. I used to have this problem to and every now and then I still do. The more you get used to approaching random people and opening your mouth to them you'll get better at conversation. As K pointed out, the main cause of the stumbling block is worrying about what you're going to say will be percieved as cool or not by the group.

I also believe that running out of things to say comes from a mixture of nerves (which will ease as you improve) and analysis paralysis by either trying to remember canned material or second guessing what you're about to say.

When you're comfortable with someone you don't tend to run out of things to say. Like me I bet you can talk for hours even about lame ass shit with your best friends. Comfort is key, get used to talking to strangers and you'll be able to 'shoot the shit' with random people more comfortably.