I check total timing on MSD's @ 3500+ rpm. Anything below that will give you odd data.
I had a Snap-On light that was flaky as hell. Sometimes it was ok, other times it was intermittent, flashing once every 5 or 6 revolutions. Drove me nuts. I di the wiggle test on the pickup cable, no help. I started moving the pickup from place to place on the plug wires. I could make it better or worse, with no real correlation to position.
So I took the pickup apart. The ferrite core the pickup coil was wound on was cracked. Apparently it had hit something hard enough to do that without damaging the plastic case. I don't recall ever dropping it, so who knows.
So, try another light. Raise the RPM to 3500. try your light on another car with single spark.
If it runs good, it is not the MSD.
B.