I've started completely over a couple of times. The novel I'm working out now originally started off as two completely separate works, until one say I said to myself "I can combine these and make both of them so much more interesting." This required a lot of change in time, logistics, etc, so I just started both of them completely over. And I'm in love with it so far (almost 100 pages in!).

I think that if your heart isn't in it, working it to death isn't good. Go with your gut. It's there for a reason. Starting over this project was the best thing I ever did for it.