Strange thing happened last night...

I was driving home from my girlfriends house last night, on the way down the hill out of her hood my car started running real rough. I didnt know what the hell had happened, loss of power car sounded like a WRX. Turned outta the hood and started going down the road and it ran fine. Got about 1/2 a mile down the road to a light and stopped. Then it started running rough again, Idling real low. Turn around and go back to her house, her brother and I take a peek and he says that it sounded like i had a plug go out, we take it out, and its ok. we test the plug wire, and its ok as well. So he says that I might have dropped a cylinder . But i wonder, If I had dropped a cylinder it would have ran like shit the entire time, not have been fixed momentarily then run like shit again?? So maybe a valve got stuck? Any ideas from the mechanical wizards out there??