Importing large content with Feeds and cron
Are you running the actual site cron? If not, run it.

As for the Ajax errors when running a manual import, are you sure that it isn't continuing to import despite these? My large imports will time out the browser connection when I manually import in feeds, but they will actually still continue to process.

Also, could you try changing your setting from "As often as possible" to "Every 30 minutes", to ensure the crons aren't running over each other?