Four stroke's aren't really a problem IMO. If yours is that much trouble, you have something dying or malfunctioning. Once I had my 200x set up close to what it was supposed to be, it ran fast and ran well. I would suspect more of a 2 stroke to be unreliable compared to a four stroke.
What ever you choose, I hope you enjoy it. Be a 250cc 2 stroke or a 350x. They're all good choices :cheers: