Hi Mulveyja,
If you have our source code, then getting a debug build of the libraries would allow you to step down in our source and see exactly what line/object in that method is triggerring this exception. That would be the quickest way to see what is causing this. If you can upload a sample showing the problem, we can debug it here.
Are there multiple threads involved in your appilcation? If so, please check the Knowledge Base(KB) article link below.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=78
Intermittant errors can be indications of threading issues. Any calls into the grid must be done on the thread that creates it, and you can use grid.InvokeRequired to check for this.
Thanks,
Haneef