Try using a fully shielded SCART cable. These are usually much fatter than standard cables as all the cores carrying each video signal have internal shielding. It could also be a grounding issue, or perhaps a bad ground on the SCART socket (dry joints).
Have a go at switching between composite and RGB (if supported). If it's only noticeable with composite, open up the SCART lead and check to see if there's a 75 ohm resistor on the composite feed. Most decent SCART cables will have termination resistors in place, but some cheaper cables won't.
Also, I know it obvious but make sure the plug is pushed all the way home (at both ends). With some older sets, especially, it often took a few attempts to get the SCART plug to fit properly (and stay in), as the socket was recessed instead of being completely flush with the casing.
I'm guessing they'd rather you purchased their own cable that fitted perfectly, of course..but cost an arm and a leg!
