iTunes says iPhone over capacity and won't sync

Suddenly iTunes is telling me my iPhone (4s) is 12G over capacity when the phone shows over 15G free. Consequently, I can't sync after adding anything new in iTunes. This seems to have something to do with the music management section. After performing a complete restore, in the "summary" section of iTunes for my phone, it shows the correct amount of free space. However, when I check the "sync only checked songs and videos" box, it suddenly shows that I'm over capacity. I also have unchecked "iCloud" in the backup and selected "this computer" instead, as I suspect this has something to do with iTunes reading my iCloud capacity rather then the true capacity and free space of the iPhone.

I've restored several times, removed unneeded apps, and done everything I can think of. Any suggestions?