There was some problem with old server, and it made a lot of cron jobs failed. I'm working a lot on it, but cannot solve the problem. My hosting provider had no idea, too.
So, I decided to move to new server on Mar 04, 2010. New server is hosted at KnownHost, California data center. It has two IPs: 18.104.22.168 (primary IP) and 22.214.171.124 (secondary IP).
New server had been set up completely, and you will see a nice improvement on cron jobs executions.
If you have any question, feel free to contact me.