Hi Alexa. Thanks for contacting iOS Device Recovery.
After going through your problem, I have found that you have:
- Performed factory reset.
- Force restarted your iPhone.
- Uninstalled app that you though might be causing problem.
Now, try other solutions mentioned-below to fix iPhone 8 keeps shutting off and restarting itself issue:
Remove and Re-insert the battery: Do not do it by yourself as removing the battery yourself can be risky and hence, it is suggested that you must carry your iPhone to professional technicians or contact Apple store.
Charge Your iPhone: Sometimes due unavailability of enough power source your phone is not able to bring up the startup screen and hence your iPhone 8 keeps restarting itself.
Try To Restore Your iPhone with iTunes: Connect your iPhone to and launch iTunes on your computer. Your iPhone will automatically get detected by iTunes, click on “Trust this computer” and follow the onscreen steps to connect your iPhone to iTunes. Once connected, go to Summary tab, click “Restore iPhone” and then click on “Restore” to restore your iPhone to factory settings.
I hope your problem of iPhone 8 keeps shutting off/restarting itself would be solved now.
If your iPhone 8 is still restarting itself then find out more solutions to fix iPhone 8 shutting off/keeps restarting itself issue.