Hi @hollyhunter Welcome to the Samsung community 
Ensure all your apps are up to date. When you had your Device in safe Mode do you experience the same issue, if did not experience it then would pinpoint recent third party apps which have been installed. sometimes it can be due to a lack of memory , go to Device care -optimise now. Ensure that apps you do not use regularly are in sleep Mode.
you could try clearing the cache partition though a Factory Reset would usually help.
1. Turn off the device.
2. Connect To Computer , other Device or TV USB, Press and hold the Volume Up key and then press and hold the Power key
3. When the Android logo displays, release both keys.
4. Wait until the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.
However you may need to send an Error Report via Samsung Members. Go to Samsung Members-Support-Send Feedback-Lockup/Restarting. Detail the issue and ensure the system logs are attached. (Send system logs should be ticked though).
