On the affected channels can you get the signal strength & quality figures & post them back.
Don't necessarily assume that an attenuator is needed as it maybe a different issue.
Also where a signal strength of 70% was mentioned earlier that was for the previous version of the Youview software and with the rewrite to the current version that calculation was significantly changed. In my setup my previous reading of 65% went to 92% with the rewrite.
Anyway post back your figures & this can be taken further.