Why does my iPad dies after a certain battery percentage?

I have a 32GB iPad 3 with iOS that were updated quite recently to iOS 8.4.

I had to admit that I haven't use my iPad for quite some time. So when I started to use it again this week, I encountered this problem. Earlier this week, I charged my iPad all to 100% in the morning and then disconnect the charger, then let it be until I look at it during the afternoon, and still seeing it at 100%, but then late in the evening, it was already dead. I thought to myself that I maybe imagining things.

So yesterday, when I connect my iPad to my PC to sync up my iTunes with my iPad, iTunes then update the iPad's iOS to 8.4. After a few hours, when my iPad is at 70%, it suddenly shuts down. When I restart it (holding down the sleep/on button), it shows the dead battery screen. I connect it to the charger and then it can be turned on normally, and I see that the battery is at 70%. When I disconnected the charger,and use the iPad, the problem reoccurred when the battery is at 67%. I recharged it to 100% and repeat the same thing, only for the iPad to die when the battery is at 67%.

I looked up on-line about this problem and tried some methods. I tried the holding sleep/on and home button together method but it didn't work. Then, I use reset all settings and still the problem occurs. Then, I chose to erase all content and settings but to no avail. Now I'm dumbfounded on what to do.