iPhone 3G will only charge after software restore?

I was given a broken a Iphone 3g roughly 1.5 years ago. I replaced the screen digitizer and the battery and have lightly used it as an ipod ever since.

It has an odd quirk though. Seemingly randomly, it will stop taking a charge. When plugged directly into a USB port it will report as charging for a few seconds and then report as merely plugged in. It will continue to loose power though, and will eventually go into the black screen empty battery mode.

However, if I restore the system software, the device will then take a charge as per normal. It will continue doing this until the next random time when it won't. Does anyone know what might be wrong with it?