If your running a custom rom have you tried calibrating it lately?
If you are rooted, download an app called "Battery Calibration" By NeMa from the play store. Recoveries will do this too, but i like the app better. Run down to shutdown or 1-2%, charge up to 100%, open the app and follow the directions.
On the first discharge run to almost dead again and charge back to 100%. This is not necessary but i do it only on the first discharge. Always takes care of battery issues.
If this does not help, it might be time to replace.