Thanks for the tip about the Triad power supplies. I did a little research and it sounds like these are highly recommended and available for a number of consoles. I'll have to see about getting one (or some).
I have now done some tests with the ADC levels on the Framemeister and, just as both Mobiusstriptech and cybercylon suggested, the ADC was a source of some of the noise. Adjusting the ADC levels (lower to mid-90s from around 128) does indeed reduce the visual noise in the SSDS3 menu to barely perceptible levels. (This was the noise shown in the screen shot I provided.) However, the ADC level adjustment does nothing to address the particularly bad noise I'm seeing during Dracula X cut-scenes (this should be visible in the videos I provided on the Google Drive). Since the noise pattern here is different (diagonal and scrolling) from the noise in the SSDS3 menu (and unaffected by the ADC levels), I would guess the root of that noise problem is something else. I intend to test with a Sandisk (recommended brand) micro SD card. (I can't help but shake the suspicion that SD card activity may somehow be involved after all, since the noise pattern changes with the game and seems particularly bad with CD games which (based on the blinking activity light) have heavier SD access.) Testing with a new power supply and/or HD Retrovision cables (which I think are out of stock right now) will take a bit longer as I have to order those parts.
I also have not checked the RF shields. If one or both are missing, however, I'm not sure what I can do about that.
Thanks for the help so far.