Hi,
It reads to me like your problem is fading hybrid vigor. The book I referenced in post 13 explains the results of fading hybrid vigor and how to breed to avoid it . With a time tested breeding plan illustrated and explained.
This plan can also be used tor restore fading hybrid vigor.
Vigor is vigor whether it comes from maintaining it in one's own gene pool or seeking it out by adding new "blood". Judge Card's plan can be used for either. The neat thing about this program is how it can be used within a strain and (with wise selection) keep on indefinitely without outcrossing to a different strain, variety, or breed.
Take time to read his book. It's short and so full of wisdom. I go back to it repeatedly when I have questions.
Best Regards,
Karen
BCM's aren't the only dark egg breed. There's Welsummers and Pendescendas too.