I am having the same problems which accumulated in a complete stop of the site Euro2012Twitter.com . The wp_postmeta contained 7.800.000 records!
Some details
1. FWP version 2011.1019
2. WP version 3.3.1 and since today 3.3.2 (same problem)
3. feed causing hash problems:
http://www.guardian.co.uk/football/euro2012/rss
http://www.goal.com/en-india/feeds/news?id=477&fmt=rss
4. feeds NOT causing hash problems:
http://www.meetyourgoals.nl/news/feed
http://2012euro.pap.pl/API/pl/Cms.Informations/CategoryRss/140/18/English%20News%20PAP%20Euro%202012
It looks like it’s not happening with all feeds but if a feed goes wrong it affects all post.
Hope this helps you a little.
rgdrs,
Cees