2004 Pontiac Grand Prix Engine failure

Engine Performance problem
2004 Pontiac Grand Prix 6 cyl Two Wheel Drive Automatic

I was driving at highway speed last night for about 40 minutes when the engine just started to slow down its rpm rate to the point where I had to pull off the side of the road and shut down the engine, which was then only idling roughly. I waited for about 10 minutes, anticipating that I would need to call for a tow but started it up hoping I could get to a station. It started fine and rode fine all the way home - another 10 miles or so! No oil light, no overheating, no leaks; nothing apparent was wrong and today it appears to be fine. I'm sure it isn't and don't want to chance it. Am I looking at a fuel pump problem perhaps? Something electrical? Would like to have some idea before I take to someone who could easily take advantage. Thanks.