Hi dklink,
We'd like to help offer some guidance that may help. This is a user-to-user forum with users like yourself, and we'll help guide you in the best direction. With what you are describing, unexpected shutdowns typically revolve around the battery performance. Looking over iPhone Battery and Performance - Apple Support under "Preventing unexpected shutdowns" this goes over things to check. In addition, checking the Battery Health in Settings > Battery > Battery Health, and verifying this is above 80% would indicate the battery does not need service. If you are above this and looking at the article, if these shutdowns continue, you will want to reach out to Apple Support
Thanks for using Apple Support Communities.