This is a sign of battery failure, most likely one of the cells has gone bad. I had the same issue in 3 of my families 4 (launch day)-Iphone 5s. I got all of mine replaced at the Apple store under the Applecare warranty. If you are out of warranty they will replace it for $80 in about 30 minutes. When I took mine in, they gave me a url to run an in-store test on my battery and all three phones indicated failure.
Since replacing it back in November, I have not seen any of the large battery percentage jumps like the many have mentioned, but I did get them constantly before I had it replaced. As many other threads on here will state, they think it is an iOS7 bug, it is not. Apple had a lot of crappy batteries installed in the iPhone5 and neglected to let anyone know about.
So go get your battery replaced and you will be a happy camper.
....And just in case, do an Itunes backup before you go. One of the batteries they were replacing broke a connector on the phones logic board, and they just replaced the phone for free. An hour later I had the "new" phone restored, and all was good.
