SQL Server unusual behavior on LOB insertion
If your SQL server trace is not showing anything, it is most likely on the application. Even if it is resolved by restarting the SQL service. I have faced a very similar issue in the past and the connection for the upload process was the culprit. Restarting the SQL service basically gives you another attempt at reconnecting. Since it works some of the time, this second attempt may be all that you need to get it going.

Look into how you build your connection for large files. Limit the time it takes to send the query to the server, and when you get past it, mimic the service restarting by trying it again.