Scart sockets and leads are not the most ideal medium for the purpose (somewhat rapidly conceived concept by the French), but using high quality Scart leads can help considerably.
Also check that Scart leads are the two-way type - some have an in/out switch for the output.
Another possibility is that the Scart switching from one device to another (it's achieved by a voltage switch on one of the pins) is upsetting the changeover.
A switchover occurs, for instance, if you turn on a VCR and start playing a video - a TV set for digital (Freeview) use has to revert to analogue to display the VCR output.
By the way, I use S-video and two audio jackplugs to a Scart lead for the DVD player rather than Scart to Scart.