Focus engine sputter

Today I was driving my focus and the engine started sputtering and then it started slowing down. I pulled over and when I came to a stop the engine died on me. After I started it back up I drove for a mile or two and it did the same thing (but I turned it off before it died). I opened up the hood and nothing looked out of the ordinary, so I just let the engine cool for about 20 minutes, then got back in and made the rest of my trip fine (about 20 miles).

I have no idea what this could be honestly. I had been driving for a couple of hours and the engine did seem pretty hot but the temp gauge seemed fine (I'm not really sure how hot the engine should get though). I had the same problem yesterday when i was driving to where I was staying for work (about an hour into the hour and a half trip to my hotel). I was on the High way, engine seemed like it lost power so I pulled over for a second and then I was able to get right back on the highway and make it to my destination.

My car is a 2001 Ford Focus, and it is due for an oil change (i'm only about 100 miles over what the sticker says).

any ideas what the problem is, or how much it might cost to fix?