Yeah, this happens to me all the time. I've managed to suss out a number of the factors for me, but based on everything else I've read on it, I've also learned that the factors, reasons, and responses are different for different people.
So, the short of it is, you'll have to figure this one out on your own, by figuring yourself out.
Here are some of the things that cause me to get lazy, and how I handle them:
1. Probably closest to what's going on for you, I will occasionally be working on a project and then hit a problem that I wasn't anticipating. It seems like my motivation goes through the basement, but what's actually going on is that I'm just chewing on the problem, turning it over and over in my head. Eventually -- days, weeks, or months, depending on the problem -- I will find the extra insight or information I needed and the whole thing will fall together. I try not to stress out over this any more; if the project doesn't have a deadline, then I let it stall more-or-less indefinitely. If it does have a deadline, I try to adjust the deadline accordingly. If I can't adjust the deadline, or it just really needs to get done, then I try to work around the roadblock by implementing a "good enough" solution. I have trouble working on project steps out-of-order, so even if I don't have to solve a particular problem in some project before proceeding, the project will still usually stall.
2. I've lost interest. In this case, the best thing is just to come to terms with giving up on the project, and I do this by realizing that there are many projects that I would actually be interested in working on, and there's no reason to beat myself up over something I don't care about.
3. I'm exhausted. The best thing to do here is go out and eat a good meal (I don't tend to cook very well), go for a walk, read a book, re-connect with some friends, and take a nap or get a full night's sleep.