I had this problem too. Even after removing the pingomatic URL it still didn’t work. I solved by going through my ping list and eliminating the ones that timed out.
My current working ping list can be found at http://www.seandeasy.com/wordpress-posting-issue-solved-at-last/
I too had this very issue. After much trial and error I eventually solved it. I have blogged about the solution here
http://www.seandeasy.com/wordpress-204-and-wpcron-database-backups/
