IIRC, multi-spark only happens until 3000 RPM then its just a longer duration spark. make sure your yellow/white (retard) wire is taped off. try new coil.
Yellow and white is the stage input wire the (rev limiter) brown and white is retard wire and it is taped off.
I'm thinking coil as well, but what's throwing me off is how it's testing fine when I check the primary and secondary resistance with my multimeter.
and it's also strange that the coil would only be affecting a couple cylinders, wouldn't it be affecting all the cylinders, it sounds like it's missing on one or two cylinders but not all 8.