Yesterday I was watching a video on my andorid GTP3-100, and my batter was at 97% after 15 min I looked at my battery it had gone to 17%, and it was draining battery fast and after 5 min its went to 7% so and I had no application running in the background I usually charge with my usb wire in my office . I have also noticed something strange since I have started to charge with my usb wire suppose the battery is at 77% if i plug in the usb wire to my pc within 1 second it goes to 100%.
I believe it is your battery which is at fault. I've seen this problem as well. How old is your phone?
Also, you may try charging the battery separately once from an external charger. If the problem you mentioned persists, then you should get yourself a new battery.
By process of elimination, i would try a different charging cable, failing that backup and factory restore? If still not working you probably have a worn out or faulty battery, replace if removable. All else fails could be your charging socket or board related, in that case bin it and seek replacement.
Agree, ive seen it on my old GT-i9100 with the battery was "jumping aroud". What you could try thou is a battery calibration.
For example this seems to do the work : https://play.google.com/store/apps/details?id=com.ignisoft.battery.calibration
(Thou i havent try this app, battery calibration have helped me before )