It's almost certainly lead build up and you're not quite able to get the engine hot enough to move the lead.
You're perfectly entitled to take the HT leads off and remove the plugs to have a look at them. Start with the bottom ones attached to the mag that you're having a problem with
.
Just be careful when you put them back on to tighten but not overtighten them and that they are properly threaded. I asked a fellow Cub owner what torque he used and his response was "Techt".
Get a wee bit of AVGAS to clean em and a gentle wire brush and remove the lead deposits. 9 times out of 10 that's what has caused rough mags for me.
One thing you can do is get a few spare plugs from LAS Aero of the type in your engine and swap em out with badly fouled plugs.
If that doesn't clear it you may need to get help from an appropriately experienced engineer type.