I've got a component cable - couldn't get the official cable, so bought a gameware component cable from game.
Strangely, this problem only started today, after I downloaded Castlevania from the VC. Before that, the VC games worked fine
with the component cable.
However, I have found a workaround of sorts, if you delete the savestates from the memory, and restart the game, it works without any problems, however, as soon as you hit the home button to go back to the Wii menu, it creates a savestate, and next time you try to load the game you get the same issue of flickering/out of sync.
Only way of going back to the menu is powering off the system.
This solution doesn't seem to work for castlevania though....
Strangley, the issue doesn't appear to effect Mario 64 or R-Type....