iPhone 4s not synching with iMac (trust this computer?)

My phone will not sync with the computer that it has been paired with from the very beginning 2 years ago. It repeatedly asks to "trust this computer?" To which I reply "Trust". It appears for a second, then disconnects automatically. It will not even charge on the computer.

I've seen that there have been a lot of threads with people having this problem with Windows 8, but I haven't seen anything regarding Macs.

Anybody have any fixes yet?

Thanks!