Do you leave your hand on the shifter while cruising? That'll wear out your synchros quick. My friend used to shift and leave his hand forward with lots of force. I freaked out when I caught him doing it. I was like, "WHAT THE HELL ARE YOU DOING?" He said, "What? this?" as he pushes the shifter forward over and over and the car's making a really weird noise. I was like, "STOP, you don't have to keep doing it!"