Why does Granville now have a stammer like Arkwright had?

Granville never had a stammer in the original series, its was Arkwright who had the stammer so why is he stammering now like Arkwright used to?. You don't just develop a stammer out of nowhere when you previously had none at all and was very well spoken. The stammer is not even funny like it was when Arkwright was doing it, its is just annoying.