Site crash with Ultimate Facebook

Hi, I am running a site called http://www.vinocotto.com.au with two WPMU plugins and the dashboard. Up until last week all was great, everything working fine. Over the weekend we upgraded our webserver hardware and software to the latest Centos Linux and PHP and other latest version software. 2 CPU, 4gig Ram...
After the upgrade the site kept crashing whenever loggin in as admin and attempting to do anything. After some painful trial and error we have managed to nail down to the Ultimate Facebook plug in... When this plugin is activated the site hangs to admins; publicly it works ok until it hangs in the admin dashboard then its dead through out. Eventually the servers throws an error 500. We can't seem to get any valid info out of the server logs. PHP mem limit has been increased to 256 Mb. Wondering if you could offer any help. In the mean time we are just running the Floating Social plugin.
Any ideas,
Thanks,
Dave.