2005 Nissan Sentra 1.8S - Starting Rough or Not at All

I’m having some issues with my 2005 Nissan Sentra. I’ve browsed the forums looking to see if anyone has had a similar issue but haven’t found a match yet. What’s happening to me is, the car just won’t start sometimes, particularly now that the weather is getting colder. I will turn the key in the ignition and it sounds like it wants to turn over but it won’t actually start up. I can smell gasoline on the exhaust after I do this but I assume this is because I’ve flooded the engine from cranking it for so long but I don’t know for sure. I can usually get the car started if I come back in about 15 minutes and try again. It will start up rough but drive perfect after that. Now I’ve already replaced the battery (which was going bad and corroded) and that hasn’t fixed the problem. I recently took it in to a reputable auto shop and they can’t figure out what’s wrong with it because it starts up perfectly for them every time. I fully expect the problem to reoccur since 3 times over the last 4 days that I was driving it wouldn’t start up. One time it wouldn’t start up in the morning after sitting over night, and twice it wouldn’t start after work. I’ve had this problem for awhile now (1+ years) and it only seems to be related to the weather. I’ve taken it to a couple places and they don’t know whats wrong, nor can they replicate the problem.

Any ideas? I’m going to pick up the car from the shop this afternoon because I need it for work tomorrow, but I hate having an undependable car.