iPhone 3GS sometimes won't charge with iTunes

My iPhone 3Gs sometimes won't charge if I connect it with iTunes. Maybe it's charged but I mean charge a little. For example, when I go to bed, I connect my iPhone to my iMac G5. At this time it's read 3% battery, but next day it's read 4% battery.

The issue is happened randomly. After I updated to iTunes 9.0.2, the issue seems discovered even more frequently. The first time I used iTunes 9.0.2 with iPhone, I got the issue. After reboot my Mac and iPhone, it worked fine and I got 100% charged. However, the next time which was yesterday, I realized the issue again this morning. Does anyone have the same problem just like me? Any suggestion? Thanks!