you may have a loose connection on one of the ends. If they are low quality scart leads they can be affected by bad connections. You can get gold plated scart leads that provide a better connection with less loss but these are more expensive as you would imagine.
Try pushing the connector into the socket as much as possible and if it still has the same problem, then I would recommend a new scart lead.
I reckonm that this has been the issue in the past and the engineer has thought it to be an easy job to fix as the first thing they normally check are the connections. Currys and Sky are laughing from the money made on calls like this so head off as much as you can by checking the connections first.
I hope that this helps.