I've experienced errors with the Captcha as well but noticed a pattern to the failures.

They occur when there has been a long pause between mining stopping and activating the Captcha video.
When I neglect the window for a while I now hit 'Refresh' just to be safe before playing the video and Captcha
errors have gone from frequent to so infrequent I'm not sure there's still any errors at all.

Recommend fixing the stopping mining problem first. Without fixing that any other improvement is pointless.
You're getting a lot of new visitors because of the newness of the site but at some point people leaving out of
frustration is really going to hurt. Whatever fixing it costs will likely save you many times that in advertising in
the future. I know there's a planned fix, I just hope it's really soon. =)_