Peugeot 406 misfiring - coils replaced but all 6 not firing

Engine missing and poor power - had thought exhaust blocked by collapsed cats into pipe (as this had happened before & when removed all came good).Symptoms very similar but appears that NOT all 6 cylinders firing.

Coils replaced but mechanic cannot seem to get all 6 to fire (assuming plugs are all ok). Completely baffled: engine had pushed blue smoke on deceleration which suggested valve guide seals fail (car had not been driven for 6-9mths) but specialist engine rebuild mechanic does not think that's the case....believe misfiring may be causing blow-by.

This has been going on for ages & everybody seems utterly stumped. Is there possibility that firing signal to coil is at fault and where does that signal-to-fire come from?
Is it possible timing belt has jumped a slot...cam belt changed less than 5,000km back (by authorised Peugeot service centre)?

Thanks for any help I can find.