Fuel air spark
I agree with fuel air spark.
10 years ago we had just gotten a -6 with 0-360 FP.
Flying home from the purchase we noticed a slight miss...previous owner said he never noticed it. :-(
Try as I might I could not create it, or predict it. But it persisted non the less.
The air is easy to check. All clear? no intake leaks, compression test, etc.
The spark is easy too, mag check/inspect, wires tested, Plugs cleaned, tested.
The fuel was harder to test, at least once fuel flow was checked as we did not have equipment to test the carb. So we replaced it with a new one.
Problem solved!!
Ps to story, I sold the carb as a core with the history, new owner had it Overhauled and the problem was still there....so it was really something with the carb. Just a story to help you figure it out.
Fuel air spark. Good Luck.