This answer might not immediately address the cause of your memcache issue, but because of the unreliable nature of memcache I would suggest to make a backup of your memcached data in some persistent storage. Memcaching your data will help you to improve performance of your application but it is not fail-safe.
Maybe you can make a fallback (persistent) storage in your AuthenticationService instance. Then first you try to get your authentication data from your memcache and if nothing is found you check if there is something available in your persistent storage.
This will at least solve all issues with unexpected memcache loss issues.