Seeing as you have melted spark plugs, burned a hole in a piston and now cracked a spark plug, those are all tell tale signs of running to lean and/or overboosting. What boost is it running (keep in mind to check out the boost gage source pressure location/plumbing), what mods have been done, etc.
I would expect that you're overboosting, which is causing the missing, and leaning it out which is causing the various parts melting/cracking.
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1990 Skyline GTE/S-t 4 Door Updated 6 Mar
Swapped out RB20E with RB20DET
Trans converted from Auto to 5-Speed...
Now with SR20DET 5-Speed
Installed Type M 5-lugs and brakes, R32 GTR rims and 2-Dr rear end (Tail lights & bumper)
1994 Isuzu AMU
US Spec Amigo with JDM Isuzu MU (Mysterious Utility, as it's called in Japan) nurf bars and hatchback style hardtop.