Hi Snowwie. Welcome to AORG.
Could be the cable or the circuitry. I doubt the cable has a ferrite core on it? You can buy clip on ferrite cores at electronic components stores, that might help. It does sound like RF interference? Might even be coming from your A1200 RF Module especially if it has the shielding cover removed.
Other problems could be loose shielding connections, corrosion or degredation in the cable. Especially if its old.
It could also be a lack of signal earth on the cable. RGB likes a separate earth for each signal RG&B.
Alternatively but unlikely the pinouts might not be optimum. Check the amiga manual pinouts to the monitor's scart and ensure the correct pins are in use. As it is a standard it is unlikely that they are not correct.
Finally It could just be the quality of the monitor circuitry. That's less noticable from a standard scart video component.
If it was my setup i'd be re-wiring the cable with new high quality video cable, separate signal earths and adding a ferrite core (maybe 2, 1 at each end).
BTW your setup looks great.
