Vauxhall Zafira blowing smoke

Hi my 99 zafira runs really well for its age its well maintained, with 211000 on the clock. Once warmed up doing say 60
Putting my foot down to overtake she pulls well only to give a little shudder as if pulling through a small blockage next thing a huge clowd of gray smoke, once the clowd has burned off no more smoke no more shudder, like I said she runs really well.