Thanks, everyone. I've been using MB bottles - and some similar ones from Bitter Lemon bottles - and am very careful with the sugar (I even have a MB sugar measure). And yes, I almost always brew dark beers. It probably is the that the bottles/caps are forming cracks. I've thought about glass, but I was worried that they would shatter and send out shrapnel. That used to happen to my grandfather when he bottled his own Root Beer many years ago.
I’ve been having this problem after I’ve bottled my beer. Some of the bottles overflow after a week or so. The remains of the bottle taste fine, so I know that they are not spoiled. I am careful not to put too much sugar in. I was thinking that the problem was that my basement does not get much above 60 degrees, so maybe they were still fermenting. I kept my last batch in the keg for 4 weeks before bottling, but the same thing happened. Any ideas to help me keep more of my beer?