Fatal Wp e-Commerce Error!

Hello Community,
I am using Wp e-commerce, Wordpress Multisite, Buddypress (all the latest version) as well as a few other WPMU and standard plugins, but when I am using the site at different times it will time out and show the error below.

Fatal error: Call to a member function add_cap() on a non-object in /home/mar3352/public_html/mall/wp-content/plugins/wp-e-commerce/wpsc-core/wpsc-functions.php on line 434

It is not always the same line.
Also I have posted this on the getshopped.org forums and have had no support for a few months.
Can anyone help me fix this, please.