I think I've noticed an additional issue with my database, it was first designed in Aug/Sept 2005 (probably in Filemaker 7) and I've now worked out that this 'portal pop' issue only occurs on this particular table which is several years old (even though we now use Filemaker 9).
When I looked at a more recent table which was added more recently to the same database, I don't have that problem, so it must have something to do with when it was first designed.
This also could explain why I haven't found any recent posts on 'portal pop,' the most recent discussions were mid-2005 (when Filemaker 7 was used).
Mind you, if I'm right, it will probably be even more difficult to fix.