Well, I have to say I'm stumped.

Just to clarify, I understood from what you've posted that the SQL Agent service startup account is a Windows domain account that is part of the sysadmin role, that the SQL Agent job is owned by a Windows domain account that is part of the sysadmin role, and that you run the job manually under your Windows domain account that is part of the sysadmin role. I can't think of any reason that the context would switch to sa for any reason.