I had the same issue. It caused some other key plugins to fail. Additionally the post -> post all (wp-admin/edit.php) failed to load. I was getting massive memory overload. Its a useful plugin and works wells (otherwise). In a week and a half i had 38,000 entries in the wp_postmeta (table) with the syndication_item_hash reference. After deleting these entries in phpMyAdmin and disabling FeedWordPress. All seem to go back to normal. But unless this issue is resolve i can use it.
I want to use this plug-in and it really solves a key need. Maybe i am missing something in the settings.