I've often been through stretches where I feel burnt out at work, can't seem to pick up steam on a project, and end up 'wasting' a lot of time looking at my favorite websites and doing other non-work stuff. I've found that these end up being balanced by periods of very high productivity. It's like there is some sort of subconscious self correction mechanism at work. I used to fret about it, but now I just accept it as part of my work habits.