Some apps that you download can cause issues with your phone like restarting itself, freezing, crashing and running slow.
You might be able to fix the issue by trying the following options:
1. Make sure all apps are up to date and restart your phone.
2. Check for and install any system updates and restart again.
3. Then use your phone and observe whether any updates made a difference.
4. If that made no difference, please start your phone in safe mode.
5. Please use your phone in safe mode and observe whether that makes a difference.
6. To leave safe mode please power cycle the device.
In the case that your phone performed well in safe mode, an additionally installed app might be causing the problem described. Please remove the last additionally installed app and test your phone. If the problem persists install the app again and remove the next app until you find the app that caused the problem.