Parallel Redo Message Spam
Those &quot;Starting up database&quot; messages occur when the database starts up (is opened / accessed by a user) for the first time.  Normally you'll only see this message once (when the instance starts up), or if something happens to cause the database to shut down / close (like an availability group failover).
If you are seeing that message a lot, one likely reason is that the database AUTO_CLOSE option is set to ON.  This means the database is shut down / closed very quickly once there are no connections to that database.
Your service probably uses best practice patterns of opening a connection, doing some work, and then closing the connection.  This would lead to the database shutting down / starting up a lot if that's the only thing accessing the database.
You can, and probably should, turn this off to make sure the database remains open and online even when it's not being actively used:
USE master;
GO
ALTER DATABASE YourDatabaseName SET AUTO_CLOSE OFF;
GO

The parallel redo messages are also normal, and generally only occur when the database is starting up.  See this Q&amp;A for more information on that one: Messages about parallel redo
You mentioned this problem is intermittent.  It's possible some other person or process is changing this setting periodically.  Or that the service application you mentioned has somewhat variable performance that might lead to more or less DB shutdowns (depending on how quickly it opens and closes connections).