Hey there @scutch and welcome to our community! I certainly understand how frustrating this is and am delighted to assist you! Have you checked to see if this happens while the phone is in safe mode? The following link has steps on how to put your device in safe mode: https://www.samsung.com/us/support/answer/ANS10003495/
If it doesn't happen during safe mode, that means a third party app that is on your phone is causing the issue and you would need to go through them to uninstall them and figure out which one is making the phone do this. I would also suggest clearing your cache off the phone; and then a factory reset if you are still experiencing the issue.
In the event none of the troubleshooting steps provided resolve the issue, the phone will need to be serviced. You can reach out to myself or any of the mods with your phone's model code and serial number or you can call 1-800-SAMSUNG (1-800-726-7864) for immediate assistance.
In case this happens in safe mode too, since you appear to have a legacy S phone which could be several years old, is the battery swollen or not holding charge? Does the phone feel hot before it restarts? Could be a dying battery or thermal issue. Knowing more details could help.
