I agree with the above in that it's probably a bad app installed on the phone. Also, do you have the device maxed out in terms of apps and low memory? This could also be a problem. My suggestion:
1. If you have lots of apps installed start by removing whatever you don't use and free up some space.
2. If the above doesn't solve the problem then backup and sync what you can and prepare for a factory reset. Start fresh and re-install apps you want back on the device paying attention to what you install and if the problem returns. If it does then it's a good chance the last app you installed is the culprit.
Here are some resources to backup the phone without root (assuming you don't have root):
SMS Backup
Call Log Backup
App lists backup
Please note that you need root access to backup app data. The above will only allow you to backup a lists of apps you currently have for future install
without data.
Finally, remember to sync you Google account to backup contacts, email, photos, etc. Good luck