....I was about to post the exact same topic.
- It's not the charger, have tried about 3 different ones
- It's not the battery, have swapped with a friend
- It's not the firmware, factory resetted and have never rooted/played with it.
Have a feeling it could be water damage, but only from rain, hasn't been dropped.
If I scratch the USB bit in the phone where the charger plugs in, it seems to start working for a bit, might last a day fine, might be five minutes.
Any ideas?