Thanks Drew - Not wishing to tempt fate, but I think I have stopped this behaviour. (delete and start over).
Just to say, the Collection Item in the Footer Symbol twice did this. On both occasions, it was pulling content from another Collection higher up the page.
On each occasion, different collections (and their content) were incorrectly appearing.
When I clicked on the problem area in the Footer Symbol, my cursor immediately jumped up the page to the wrong collection.
Sounds like the collections had crossed wires.