03 altima crank but wont start HELP !!!!

I have an 03 Altima. It cranks well... but sometimes it won't start. I had to have it towed to the dealer this last time. The code came up "misfire" is what the dealer told me. And said it was a very generalized code and could be anything. They replaced the spark plugs and it worked great a couple of days. Now, it's back to not starting well. Any ideas?

Dude again it the ecm/cat/head gasket. My guess is Oil comsumotion or coolant consumtion(sorry abou spelling this ain't english class) if it starts it will run well, until it burns what's in the chambers;but trust me take it to get reflashed if you have not done so. From what i have heard and seen they reprogram the ecm to ra=ead the crank/cam sensor as one, worked for me. Either that or they will replace the engine, cat, and reflash the ecm for free if you are lucky, BUT DONT WAIT!!!

I bought my 03 new and it's done this about two times a year since probably 2005. The dealership told me it was a common problem in the 03 but they didn't have a fix. They told me to step on the gas and let it crank until it starts. This always work but sometimes it takes a few minutes. When it does finally start huge clouds of white smoke come out the exhause for a few seconds and then all is fine. I know this isn't a technical answer but it's all I have. Good luck.