People have problems with playing complete operas in iTunes. The software just reacts to the text that was imported. I usually find that I have to change the text. For instance, if I want only one entry for each composer on my iPod, I change the composer's entries to all be exactly the same. Warning: I have a really old iPod, but my iTunes updates regularly.
To edit the text on iTunes use Control-I key combination to bring up the edit screen. Then click the Info tab.
The Album Artist and Album fields seem to control what plays with what. If you want all the tracks of an opera to play without stopping, change the text of these fields to all be the same for that opera. Copy and paste. You may have to spend time finding all the entries you need to change.
Over on the right side will be boxes that say Track number and Disk number. These will control the order anything plays in.
Be sure to save your changes after you make them.
Click on disk 1, track 1 to begin your playback. This works for me. Most of the contents of my iTunes are ripped from CDs.
My sense of this is that this problem is a relatively recent phenomenon resulting from one of the iTunes upgrades.
Changing the text also fixed another problem I had with iTunes: I bought copies of Die Schöne Müllerin by both Matthias Goerne and Jonas Kaufmann. Because they both had exactly the same album name, track one of both albums would play on my iPod before proceeding on to track 2. This was amusing for only a while. The solution was to change the name of one of the albums in iTunes.
I hope this helps.