I had a similar problem recently. The memobj_indexstatsmgr of memoryclerk_sqloptimizer consumed most of the memory and caused sql server to emit message "There is insufficient system memory in resource pool 'internal' to run this query" in the error log. We could see it gradually creeping up using perfmon. It was 2008 R2 64 bit that we used.

But after we applied the service pack 2, it no longer crept. Refer to this bug list http://support.microsoft.com/kb/2528583 , fix of id 657405 seems relative, but I'm not sure.