2001 senic running rough?

i have a 2001 y-reg senic 2.0l auto, it is running like only 3 plugs are fireing but these are ok, it sounds like its got a blowy exaust but that is ok too, when its ticking over its like its missing but seems to drive ok'ish as it sounds blowy and seems like it drags abit.

it also drinks alot of fuel, i have had it to a garage were they changed a coil as it wasnt working, now when each one is removed the engine dies down so these must be ok.
they said it could be the cambelt has been put on a tooth out and theve checked this and it has been on the computer and no codes showing.

they now are saying it might be a locking nut that is on the end of the crank that might not be locking.

would anyone have any idea if this could be right or any suggestions to what might be wrong?

thanks
kevin