We recently had the same problem on a Homelite 25cc magneto equipped engine. We shielded the plug and mag wires and it went away completely. If you have some old television coax cable, you can carefully strip the copper braided shielding off the cable and use that. Ground only one end of the shielding, not both. In other words, run some of the braid, or a wire from the braid to ground on the engine at the spark plug, but don't ground the other end near the coil. Do the same on the kill wires. This changed us from having uncontrollable glitching at a 20 foot range check, to having none at all at 150+ feet.

Good luck,
AV8TOR