iTunes is freezing and I don't know why

It used to work but now a tune will play a little more than a minute at which point the progress bar freezes. The tune will continue to completion but the next tune will not play. I have to force quit the app. It does this on smart playlists and on selected tunes from the library.

I had 11.0.4 installed so I booted from a backup, deleted the app and the iTunes folder in my library and re-installed 11.0.3, which used to work. It does the same thing.

My iTunes library is on an external disk as is the iTunes media. This has never been a problem before. However, I notice when iTunes start it frequently finds the itl file to be corrupt and tries to rebuild from the xml file. When I copy what should be the identical itl file from Previous iTunes Libraries it always fires up correctly, or at least I thought it did. I don't know if this is related or not.

It strikes me that once iTunes finds the tune and starts to play it that the iTunes library should not matter, but maybe it does.

Any ideas?