Mythtv : downgrade to 0.27.4

Hi,

I'm suspecting an issue with mythtv 0.27.5 on the backend component.
How can I downgrade to an older version of the package mythtv-backend
and its dependencies to 0.27.4 ?
Here is my sources.list :
deb http://www.deb-multimedia.org jessie main non-free
deb http://www.deb-multimedia.org wheezy main non-free
deb http://www.deb-multimedia.org wheezy-backports main

Best regards
Guillaume