Try a hard reset on your phone.
Be aware a hard reset will erase all your downloaded apps and user data. It will restore your phone to a factory default condition. It will be like when you first bought it.
Before you perform a hard reset:
1. Backup the phone using the phone's backup feature.
2. Ensure that the battery is charged.
3. Do not leave the charger connected when you perform a hard reset.
4. Remove the external SD Card (if applicable)
Here is a link that shows how to perform a hard reset on your phone. Try the second method first.
Be patient it takes a little while to complete the reset.
Once the reset has been completed, test to see if the phone performs correctly. If it does, restore the phone using the backup that you made earlier. Use the phone's restore feature.
If the phone still performs correctly, the reset has corrected whatever was causing the problem.
If it once again has problems then it is one of the downloaded apps (not the pre-installed ones) that is causing the problem. You will then have to determine which one it is.