check compression like vr6 said, if that is ok, id strip and clean all your breather hoses, air box assembly, bang a new filter in and try that 1st, if still the same you could try some injector cleaner like reddex etc, ive used it in both of mine but i havent had a problem to start with, so i wouldnt like to say that it would work, the problem could be down to a number of things but try what vr6 said 1st, if its a tired engine that may be why you have probs