XML Docbook validators?

I'm trying to find a good XML/Docbook validator that can work in a mixed 
environment where I have project where 90% is docbook but also 10% based on 
older XHTML which is validated against DTD's  (for example   
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" )

The problem I have is that while the docbooks validate fine as soon as I try to 
use any of the W3C DTD's they will fail since they use the syntax (for example)

<!ENTITY % ContentType "CDATA"
    -- media type, as per [RFC2045]
    -->

which will fail to parse since the validators think that the ENTITY element is 
missing an ending ">" and don't understand the "--" as a comment.

Is there anyone who knows a good validator for both Docbook and older DTD's ?

(I've tried Xerces and Sun's MSV)

Cheers,