Interesting enough there was a guy on TV today talking about hijack scenario like an inside job, and how they could try to avoid radar and that most likely it would require someone with really good working knowledge of the aircraft, not someone had just flown a simulator (which I assume was a reference to the WTC attack). I know of no evidence so it just sounded like he was running what-ifs through his head.
This is one of the stronger guesses imo. At night, over the ocean, possibly an undercast (although I don't know what the weather was), electrical/nav systems go kapluey, possibly related onboard fire (something similiar that happened to
Swissair 111) they get lost and eventually run out of gas. As I recall the Swissair flight crashed before it ran out of gas. There the crew spent too much time messing with the issue instead landing immediately.