I suggest you power down the phone first, and run it through a full charge cycle to 100% with the phone off. Cycling can be due to an artificially depressed battery voltage. This happens when the charging and metering system gets out of sync with the battery's actual charge level. This fools the charging and metering system into thinking the battery has more power than it actually does. By charging with power off, you force the charging and metering system to set the values back to what the battery actually has in capacity. Then you should use the phone normally until the low battery warning appears, then power off again and repeat the charging to 100% with power off.
Once done, the phone should act normally again.