404 sometimes goes to host 404

[ferby]

If I go to mysite.com/thispagedoesntexist then GS will show me Oops page not found 404 page. But, if I go to mysite.com/admin/template , instead of getting the GS 404 page, it goes to a 404 page at my web host's domain (?!)

If I go to mysite.com/theme/themename/ I get the host's domain error 404. But if I go to mysite.com/theme/themename/folderthatisntthere I get the GS Oops 404.

How can I force the site to always use the Oops 404 on my site and never use the host's domain 404?