JVM3670SF001 microwave failed again after repair

[DEMinSD]

I'm so disappointed.  Our GE microwave JVM3670SF001 failed a few weeks back.  When using, suddenly heard some noise that sounded like sparking/arcing inside the microwave (not where the food is, but inside the microwave).  Afterwards, the microwave would function except for the fact it would not heat the food.  Everything else worked, and sounded normal.

Did some research and decided I needed to replace a diode and the magnatron.  Sourced the parts and performed the repair (myself).  Did a test with water and happily, the water was hot!

Reinstalled the unit over the range and the microwave worked for a few days, reheating dinner places, etc.,  until last night.  Then, same disappointing sound -- sparking/arcing sound inside the microwave.  I stopped it, and tried again a few times, and as soon as it would normally start heating (you can hear a difference in sound when heating and when just running), there is a sparking/arcing sound inside.  

I'm guessing it's coming from the new magnatron but maybe not.

So, the big question is, what else would cause the repair to fail within a few days?  Not sure if the new magnatron is still good or not.  It still makes the sparking noise so it's not completely dead (yet).

Advice appreciated.  Since it's a 36" unit that is no longer made, repair was our 1st priority over buying a new 30" and an over-priced 30->36" mounting kit.

Thanks.

[MrApplianceMatt]

Is there any enamel damage on the inside of the unit? Any bare metal in the unit will cause magnetron damage. The main parts of the high voltage circuit are the transformer, the capacitor, the high voltage diode, and the magnetron. Microwaves are not for the faint of heart and can absolutely kill you if caution is not exercised.

[DEMinSD]

I checked and do not see any enamel damage in the unit.  The fact that it worked for a couple days is baffling to me.  If the capacitor or transformer were bad, you'd think it wouldn't have worked after I replaced the magnatron and diode. 

I do understand the danger of working with the high-voltage parts like the capacitor and take measures to discharge it before working with it (like when replacing the diode).

Since the diode and magnatron are new, would a bad capacitor or transformer cause a slow failure of a new magnatron?  Do you think the new magnatron is now damaged also?

I thought of calling an appliance repair person, but I know they will charge me retail price on the parts and the repair would wind up costing me more than the microwave is worth fixing, hence me trying to fix it myself.

What is my next step, if it were you?

[MrApplianceMatt]

Some repair places will install a customer supplied part with the understanding that there's no guarantees. One thing you can check if you haven't already is under the plastic cover in the ceiling of the microwave that usually says DO NOT REMOVE. Under this cover you'll find a free-spinning metal fan blade whose purpose is to scatter microwaves in the unit more efficiently. Years of usage can lead to buildup of vaporized grease and other yummy stuff under the edge of this cover, regardless of how clean the inside is kept.  I've serviced units that have actually caught fire here because of this.  Also check the turntable wheels for cooked on, carbonized food.  Carbonized food can cause a similar reaction to having a metal spoon in the unit.

[asteysn2]

iD ALSO LOOK FOR BARE METAL INSIDE THE WAVE GUIDE. THERE IS PROBABLY SOME SORT OF Mica covering the outlet of the wave guide. Every thing there has to be clean and no bare metal surfaces