Try outdoor instead? It happens only indoor right?

I see your pix are all indoors... my hypothesis is that the lights are fluorescent lighting which flickers at 60hz, so at higher ISO you use higher shutter speed and you capture different lighting each time.

Could it be amp noise? Perhaps a long-ish exposure at high ISO with the cap on and the rear viewfinder blocked will reveal that? But if it really is amp noise that bad, you gotta send it back.