I can't write stop-and-start, which totally hinders me because that means I can only write if I have huge chunks of free time...which, let's face it, I definitely don't have! Do any of you writers encounter this problem?

I think I get what you mean -- once you start writing non-stop, you get completely in "the zone" (for a lack of a better word) and once you break out of it, it takes a hard to time get back. I used to have the same problem but I figured a pretty easy solution is just to constantly think about the story/plot/characters when you're not writing, so that when you do get to write, it's still running fresh in your mind.
- Marta