Steve, I get the impression that this isn't a foundation problem.
But then, I've been wrong before. :o)
Dan, here in the north, we have a problem called "Truss Uplift". This is similar to what Steve is discribing, only with the house itself and not the foundation and ground. What happens is during the summer when temps and humidity are at the high, the wood frame of the house is expanded (Like Steve said, similar to a sponge). Then along comes winter and the whole thing reverses. Low humidity and temps and the whole wood frame of the house contracts and Viola, Lots of cracks and, in some cases, huge gaps. We had one house that had a ceiling lift almost 2 inches every fall and drop back down in the spring. We ended up tacking a moulding along the ceiling, tacked to the ceiling joists only so that when the trusses contracted, it took the moulding with it and the gap stayed concealed.
The bad news here is that the framing of the house is probably at fault. Without a major repair, there may be nothing to do about it. The good news is that, other than the cracks, your house isn't going to fall down around you during dinner one night. Still, I'd seek the advise of a structural engineer.