Ipod not loading on iTunes/macbook

When I plug in my iPod to my Macbook it usually does not register and show up as being plugged in. I then have to unplug the iPod, and it is frozen on that screen, forcing me to restart it (holding down circle button and menu). Once I do this and then plug it back in, it finally registers on the computer, and I can see it in iTunes. This has been happening for quite a while, I took it to the mac store, and of coarse it did not do this when he tried it. He suggested a new cord which I have tried but that doesn't work either. I would say this usually happens 90% of the time I try and plug it in, but sometimes it plugs in just fine (with the same cord). Any thoughts on what's going on? I assume having to force restart the iPod every time is not a good thing for it.

Thanks!