Lumen Authentication with Multiple Requests from one client

I've built an API for a mobile app to get data from our database. The app developer uses a client ID and secret to return an Oauth token, which then allows them access to the API. We've discovered that when they make multiple requests quickly, like 3 in 150 milliseconds, they get 500 errors.

In my log, I have a bunch of these: lumen.ERROR: exception 'Illuminate\Auth\AuthenticationException' with message 'Unauthenticated.'

I've done a bunch of Googling, and cannot find a solid solution. Can someone point me in the right direction?