I suspect that an upsurge of pr3 inbound links (themed), may have had a negative effect on my pr.
Interesting. This gives me some food for thought.
As ones website rises in PR, those low PR internal pages begin to achieve some PR, also. This begins to theme your website from the bottom up. This may be very good if those internal pages are on theme. Very bad if they are haphazardly put together and off topic.