B&M's never stall, they are designed to go through their elements quite fast. However all roller coasters can have their speed manipulated by wheel compounds.
Slightly off the point of Nemesis, but PMBO has different wheel compounds on the three trains it has got and on certain days, if it is running two, you can usually tell one train from the other.
I am not sure who made the point about Nemesis being a bit tamer at the start of the year and then getting better as the season went on, it is a trend I have noticed at Towers in the past.
I am really unsure when I'll next get to Towers but I do hope it'll be soon, after getting plenty of rides in on Inferno the other week, I'd like to get a few rides in on Nemesis!