What you may find is that the scart lead which you are using could be causing the problem.
At the moment the scart lead is 2 way, Audio and Video going both ways. What you are best doing is to cut pins 1,3 and 19 from the scart plug which goes into the TV.
This makes the scart lead one way and also prevent "Cross Talk" from the audio and Video lines.
What happens is even though you switch the TV to "AV/Scart" the television is still receiving TV programs which are sent out of the scart socket on these pins which you should cut. 1and2 are audio and 19 is Video.
If the problem still happens it could be internal break through in the TV set, and if thats the case there is nothing you can do.
Good Luck
Delius