The final fix was the sun/moon graphics glitch on the left/right sides of the frame, invisible on the ZX Spectrum because of the video attributes in use. For the Coco3 port the easiest fix turned out to be simply manually extending the frame sprites, with respective masks, which (IIRC) is what the Amstrad CPC port (and likely others) do.
|No more sun/moon graphics glitch left/right of the frame!|
The only thing left is to tweak the ballast code in the main loop to slow down 'empty' screens. I've made a quick attempt but will probably spend more time comparing against the ZX Spectrum version before it is finally released.
I've sent out a few copies for testing and review... so hopefully it'll appear on YouTube at some point in the near future!?!
Still undecided on release options...