Check that your points are at 12 to 14 thou. and then most of the time its the coil that takes a crap and arcs to case. I have had to replace a few of them before i went heat exchange. I have a mag with upgraded pulley and it makes it not work so hard slows mag speed seemed to really help mine. It arcing somewhere you will just have to inspect coil for cracks or black lines where its jumping out of it.
also have him look inside of the black cover and look for any cracks or arc spots as well. when your electrodes wear out they get a wide gap and makes the mag want to jump arc and once it does it will fry a path so spark will jump inside and not to electrodes.