I thought I'd throw a troubleshooting tip out there to all the beginners. I nearly e-mailed viletim for some assistance, so hopefully this can help someone else from littering his mailbox.
I've had an NESRGB board installed in my NES-001 front loader for about a month now. For the majority of that time I have had this odd horizontal scrolling interference issue I just could not get to the bottom of. It might be described as a horizontal flickering. It persisted over S-Video and RGB, but was clean over the factory composite out. It was most pronounced in games with lots of solid bright colors. Not really noticeable on dark colors or black. I have not tested S-Video (likely not resolved on my rig) but RGB is now clean as a whistle.
Your mileage may vary with other gear but for some insight, the board revision I have is an NES-CPU-11. I am connecting to a VGA computer monitor (CRT) with a passive HDMI to VGA adapter, via a Retrotink2x Classic, connected to a Retrotink RGB2COMP connected to the NES via viletim's NESRGB XRGB to SCART adapter. There are lots of cleaner solutions but I started with what I started with and ended up here. I'm sure others are in the same boat.
I tried a new 78s05, using a USB Blaster to flash different firmware, cleaning up my n00bish wiring, a variety of power supplies, etc. In the end, I found that the encoded composite must introduce enough interference to create this issue. Switching my sync line from the encoded composite (V) on the NESRGB board over (CS#) cleaned everything right up. Granted I can't get composite out of SCART at this point, but to me that kind of defeats the purpose of the mod anyway. Unless you are working TTL compatible gear, make sure to set jumper J8 for 75 ohm sync.
Once I figured this out, I couldn't believe I hadn't examined the docs and tried this earlier. Hopefully this helps someone. I appreciate the knowledge, the community brings, and thank viletim and the many other quality product developers and moders for creating such awesome additions to old equipment.