To test the mag, check for filament resistance between the terminals, should be less than 1Ω. Check for resistance from the terminals to ground, should be infinite. I suppose it could be the mag, but probably not. I've only seen them either work or not, never seen a weak one. Most bad mags I've seen have passed the resistance tests even though they are bad and shorted mags will buzz loudly. If you have a clamp-on ammeter, clamp it to the main power wire and check your amp draw. It's probably drawing a little over the fuse rating. Check your AC voltage while it's running. Stay away from the high voltage side of the inverter, that voltage is deadly. If everything checks out, I still think it's the inverter board.