What EFI are you running? Most modern systems will no longer have a rotor arm or dizzy.

The reason is most likely to be a too rich mixtue - the causes of this could be almost anything, from a warn cam lobe, to dodgy injector, wrong plug gap,bad set up...it could even be a sticking or leaking valve. Best to get it checked over by a pro - with the EFI there's not much you can do at home.