I spend some more time on this matter to try an narrow things down.
1. @leewrigley I never noticed any kind of blurriness and did not need to adjust any settings on the OSSC to get a proper picture. Just the interference pattern was a problem for me. Also this problem seems to only occur with my PAL N64 with s-video (multiple cables + consoles tested). My PAL SNES with s-video into the Koryuu has not had any problems and the composite input for everything I’ve tested also lookst great (as much as you can expect from composite at least).
2. @BuckoA51 For me at least, this diagonal line pattern seems to stay consistent in speed and direction per session. Resetting the console will change this however. Sometimes it’s worse than other times.
3. I also got my hands on a RetroTink2X Pro recently. I ran my s-video directly into that and the interference pattern was greatly reduced in comparison. Still the tiniest amount was left, but so little that you’d only notice it if you sat 2cm away from the screen. The same goes for the Framemeister as I mentioned in my first post.
4. I ordered a shielded Nintendo style s-video cable from RetroAccess. I’m still waiting to receive the cable later this month. Kind of a pain to get hold of their cables so I hope it’s worth the wait 🙂 I’m curious if their high quality, high standards cable will do better. If not than I think there is little I can still improve in the chain.