renault laguna misfire fault code 45?

my 1995 2lt 8valve has started to missfire it will drve then missfire then seams to clear itself ive changed the leads dizzy cap plugs swapped the coil pack my code reader says rpm signal fault so i changed the crank sensor still the same you can hold it at 2000rpm the it starts dropping to 1000 and back up

disparing renault owner
January 2009