If you have anything wrong with your ignition system it will cause jumpy RPM readings.
Bad plug, plug wire, points, .........
We had one customer that finally found a bad plug wire, engine ran perfect.
You are checking the resistance of your plugs and wires during your condition inspection?
When it is jumping try running on the left or right ignition to see if it goes away.
I have a number of customer stories about jumpy RPM:
1. Really upset customer on the phone with a jumpy RPM, during his phone rant telling me how bad our unit was he just happened to mention he was also really upset with his Storm Scope. Apparently his Storm Scope was reporting Lightning strikes near the front of his plane all the time. After fixing his ignition system both problems went away, even though the engine ran "perfect and he was positive nothing was wrong with the engine".
2. Another customer had a Mag failure a few weeks after the RPM started jumping and was mad at us because our tech support person told him to replace the sensor and did not mention he could have a mag problem.
Rob Hickman
Advanced Flight Systems