A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi Jhorie,
Sorry for what happened to you.
Since you can run that flow normally when this list is approximately smaller than 250 rows and encountered 503 error message when the list is larger. That means this Microsoft Flow may meet some issue within it.
To get dedicated support of Microsoft Flow, I'd like to suggest you post a new thread in Microsoft Flow Community. It is a community that handles this kind of issue. Thanks for your understanding.
Regards,
Neo