i have an mgzr 1.6 twin cam ?

I have had the new upgraded head gasket and all the valves replaced new cam belt I have had the cam lined up on a machine I have changed plugs leads had new oil right grade for this car I have had cam senser changed u name it and it runs but it misfires and seem to have a dead spot on acceleration so you have to keep the revs up I have looked on loads of forums no answers for this it comes up with fault code this is for a random misfire. why dose no one seem to no what causes this so many people with the same fault misfire