on thusday i set off to work and 3 miles down the road my rst cabby started to missfire quite bad. friday i went a bought new plugs, leads, dizzy cap and rotory arm fitted them but its still there. When the car comes on boost it goes fine but as soon as i come of the boost its missing again, ive checked for air leaks but cant find any but im going to remove all the air pipes today (sat) and clean and refit them.

any ideas wot could be causing this?