I have the exact same problem and I'm trying to connect to a macbook. I've never had a problem syncing my music when I used windows, so yeah it very well could be a computer problem. I've done everything listed on this forum and unfortunately I've had to come to the conclusion that apple products really are **** because nothing worked. My theory is that since itunes is OWNED by apple, they have the authority to deny you access to music that wasn't purchased from them. I'm betting that's why this specific issue happens ESPECIALLY when using an apple computer. I think the best way to solve your problem is to get a windows computer and sync your iphone to that.
My anger derives from nearly all my music being deleted after trying to connect my phone to this computer. Now my macbook won't even recognize my iphone when I connect it AND I have no music AND I can't ever add more music.
Thanks a lot apple,