When a battery is going bad it will show false percentages. That is because it has access to less of the 'space' on the battery to access and use/charge. Sounds like it is time for your to get a new battery.
If the device is under warranty, then contact the manufacturer. If it isn't then you can either try purchasing and replacing it yourself, or take it to a local tech.
Just a word of advice, while they tend to be cheaper, after market batteries are not always a wise choice. They can create more problems than the discounted cost is worth. So you may wish to consider an OEM battery.