Got an Ohm meter? If so what is the resistance reading between the case and the RPi common, GND?
Ohm with and without cables connected. Connect one cable at a time and ohm. If you get less then Infinity that's a problem. When you take the Ohm measurements do not touch the case or the ohm meter leads with your hand or other body part. Ohm one part of the case to the other, infinity?
Take the unit apart, clean out the dust, even the tiny bits of dust.
Other steps, wrap the case in foil, poke holes in the foil for air flow, and hook a ground to the foil. Ground being the 3rd wire from the power plug. The GPIO pin labeled ground is not really ground, more it is a circuit common.
Get a humidifer and run it.
Also, try not to zap the RPi anymore then needed. With common room conditions you can generate 5,000 volts per inch of spark, basically, Those tiny junctions inside those chips cannot take much of such treatment.
Without knowing why you are deleting my postings, I will not know how...