1999 Questions for all other cars feels like it is cutting o

Shakes or Wobbles problem
1999 Questions for all other cars Two Wheel Drive Manual

I have a rover 211, some days when driving it, it seems to struggle when moving up the gears, even in first gear sometimes, at first I though it was my driving but it happens when my partner drives it aswell. I had my mechanic look at it and he replaced some leads but it is still doing it any ideas what it could br?

Thanks

Susan