You have already tried the best solution which is a reset. Here are just a few things you may try,
1. Remove any memory card and sim card. Reset again without memory card and sim card. Even memory card/sim cards may be a cause sometimes. After reset, use the phone with a different sim card and without installing any apps. If the phone works fine, you may try putting the old sim back(not memory card). If the old sim card works fine, then insert the memory card. Install any apps sometime later.
2. Let the phone charge overnight, even if it is already full. Then use the phone till the battery completely exhausts. Repeat this cycle two or three times and see if there is any improvement.
3. There are some battery apps in play store which can recalibrate the battery. Try to recalibrate the battery. The calibration might have gone wrong or the battery itself may be at the end of life. How old is the phone?