problem with my rover 25?

hi i have a rover 25 w reg 1.4 my problem being when under pressure the car stutters as if its missing. I have eliminated a few things changed coils, oxygen sensor, spark plugs, coil leads i've also had a diagnostic engineer out he told me it was the oxygen sensor which i replaced but its still doing the same thing as you can imagine these things aren't cheap so i am now getting desperate please help.