hi dazednconfused - what year are you in? i felt the same in my second year. i found that keeping going helped, even if my work wasn't up to standard. it eventually got there. needless to say, it took a lot out of me. a while back, i read something really rewarding. it said, there comes a point during the PhD after the euphoria, boredom and frustration phase when we should see it as just 'a job to be finished.' I have found treating my work in this way helps a lot. it's no longer my world but i still work on it, with weekly targets like everyone else who goes to work.
i'd say, take a week off and do something totally different. perhaps go somewhere. you'll be refreshed when you get back and you can just treat it as a job to be finished.