Hi,
If it works normally when the charger is connected but not when working on the battery only, then it seems as though the battery is starting to fail.
How long have you had the tablet? Even rechargeable batteries have a lifespan.
Here is a link to the service manual for your tablet. Scroll to p.28 to view the necessary pre-requisites and then the procedure to remove / replace the battery.
https://download.lenovo.com/consumer/mobiles_pub/lenovo_s8-50_hmm_en_20140828.pdf
On p.37 item #2 you will find the Lenovo part numbers for the battery and the other battery accessory parts that may be required when you replace the battery. It may be that you can reuse some of them, except the battery of course, if you are careful when you remove the battery. Just search online using the part number only, e.g. battery part # is 5B19A4657T, to get results for suppliers of the part.
Hopefully this is of some help.