I don't know exactly what's meant to happen if you use one, I just know that PAL SNESs aren't compatible with RGB so I wouldn't really worry yet. In fact it's probably a good sign that it IS RGB in that it's behaving differently to the composite cable.|
My guess about the Dreamcast is that the cable has had it... or the cable housing is coming loose from the motherboard inside the console, though I don't think the DC was particularly susceptible to this problem (it's no Megadrive). I'd say if there's no sign of the latter it's probably the cable; it depends on exactly what 'tweaking' you're doing to remedy the problem.
21 consoles, 5 handhelds, 2 PCs, 1 Mac and about 700 games from the past 30 years. Every major manufacturer and developer represented. Still a fanboy for disagreeing with you.