XSLT List View Web Part Error when Using Group By

I have a Tasks List created a tasks list in SharePoint.  The Tasks (normal OOTB template) list works fine in the normal view, however, I have created an XSLT ListView Web Part that points to this tasks list.  When I set the web part to display a view with a "Group By" rule, it returns an error:

"Attempted to use an object that has ceased to exist. (Exception from HRESULT:

0x80030102 (STG_E_REVERTED))

The list has just over 1000 items and the threshold is set to 5000, I have tried indexing the column that I am "grouping by" just in case and that has not worked.  

Has anyone experienced this issue before?  If so, what steps can be done to display the XSLT List View Web Part properly?