it can be so meany things mate iv had a nuth of my car doing this so my car is going in to REYLAND to be sorted . the list of stuff it can be

plugs
plug gap
ht leads
fuel reg
fuel pump
fuel fillter
fuel pressure
fuel pump volts
phaz + gap
map sencer
tps + it setting
to much boost on the turbo for the chip
pipes on amal valve not right
blue tep sencer

Hope some of this might help you mate