Why does my iPhone 4 reboot when it's fully charged?

(iPhone 4 GSM, 5.1.1)

Hi there!

As the title says, the iPhone restarts as soon as the battery indicator shows 99-100 percent (when it is plugged). The iPhone would not reboot constantly when it is unplugged. I have tried to change the charging port, and the battery as well.

The battery is just a cheap china-battery from eBay, so it could probably be the cheap batteries that are causing my problem. I just ordered a new OEM battery, so I will let you know if the new battery will fix this.

Or could it be a problem with the motherboard?

- I am so sorry if you think my English grammar is awful