All the problems can be summed up with "crappy carbs". It's actually a long story about what started me down that path (dealing with health issues), but I've actually put back on some weight (around 40 pounds...it's easy to be blind to oneself :().
What I've found is that when I start eating carb-rich food, my ability to stop snacking on carb-rich foods goes to zero. So I've cycled between maintaining where I eat a low-glycemic diet and not doing that and gaining. When I spoke of "biological imperative", I mean me: when I eat carbs, I can't stop. I never get full. I'm sure I'd be a Type 2 diabetic in about 5 years if I didn't make this change.
A few weeks ago, I dumped the crappy carbs again. I'm back full-on low-carb and have lost about half of that[1]. This time, I'm going to learn from my mistakes. :) I'm going to follow the Atkins notion of gradually increasing carbs until you find your "critical carbohydrate level". Since I've bounced back and forth, I figure realizing that fact will be helpful in my long-term maintenance.
1. Yes, a very large percentage of this is water. I'm OK with that. :)
On a side note, I'd love to hear what sort of issues you have with maintaining?