But nevertheless, the problem still occurs. Now, though, it seems to be intermittent - always delightful when you're trying to troubleshoot something. It seems to happen mostly when I select and play a playlist or album rather than a specific song. However, I have as yet to get a consistent pattern of behavior out of it.
Clearly it's the sort of thing remote troubleshooting isn't likely to help with. I think I'll probably take it in to the Apple shop per greenwood's suggestion. Then I can show them what the hell I'm talking about.
And if it works for them and not for me, I guess I just bend time and space too much for small electronics to bear. Hrumph.