Looking at a video of the coinop on Youtube right now....
Interestingly, although the game scrolls at 50/60, the sprites appear to update at 25/30
This is particularly noticeable on the moving platforms, although a keen eye can spot it on all moving objects
See video around here:
[ Show youtube player ]
For OCS I would consider using hardware sprites for the player, ditching the parallax, and running the game in 32 colours (16 colours for the player would be fixed and reusable in the backgrounds and bobs, with the lower 16 colours dynamic per level). Update the game at 25fps but update the scrolling at 50fps