Odd video interference

[Caseh]

No sooner have I settled into enjoying my X68k that an issue appears. When powered up, the picture is fine but after say 10 minutes of use it slowly starts to develop what I can only describe as a very minor snowy/static effect.

The effect is then amplified as I plug in a 3.5mm jack for passthrough to an sc55 and it gets quite bad. Any probable causes for this? I've swapped the  cable connecting the x68k to the sc55 but it's exactly the same, almost like it's really bad shielding but it still does it even when the phono end is disconnected from the sc55.

[Caseh]

Resolved, dodgy scart wiring! Should probably look at using VGA but hey-ho it'll do for now.