help! - interference using Sky with AV amp

Hi - I hope someone can help.

I have a home cinema system and route my Sky, DVD player and VCR though an amp (I did have an X:BOX in there too, but it didn't seem to like co-existing in the same scart box as the video out from the AMP).

Thing is, after watching Sky for a while - say 30 mins - the picture begins to break up and the colours go. As I say this only seems to happen after I've watched if for a while. If I turn the system off and try it again the following day, it's fine.

Funny thing is, that I can watch an entire DVD, it's fine. I know it's not the Sky box because the picture is fine in all the other rooms.

Any ideas?

cheers

j.