My iPhone won't stay sync'd to iTunes, It syncs but then disconnects, then reconnects. Can anyone help me with this problem??

Everytime I connect my Iphone to my computer, it syncs, then soon after disconnects, then reconnects, and so on....Does anyone know why it's doing this? It makes it hard to back things up and charge it. I checked and my iTunes and iPhone software are updated and my computer is only 6 months old....help please!