Ever have trouble writing? how do you overcome it?

I am currently writing a book, and i go through phases. Sometimes I can write for weeks on end, sometimes I can't. And it's not like it's writers block exactly, because I know how my book is going to flow. It's just, i think i get tired, or weary of writing. Then, if i leave it off, and don't touch it for a while, more often than not, I come back writing with a vengeance! This ever happen to anyone?