right thats a clearer picture.
Firstly if one plug is sparking 10 out of 10 then the coil itself is fine as its the same one for both plugs so that can only be the Lead, Plug Cap or Plug itself.
Have you tried swapping the plugs round to see if the problem switches sides? that would confirm/eliminate the plugs.
Next try the same with the Plug caps and finally, as you think its an aftermarket coil, will the HT leads unscrew at the coil end? If they do and neither of the above 2 ideas give results i would fit new HT leads.
Let us know