It's not quite in the same league as any of this, but when I was a child, we sent our Commodore 64 in for repairs several times because it started "writing" by itself. Gibberish would slowly appear as if someone was randomly hitting the keyboard.
Each time it took several days before they repair centre got to it, and they then contacted us to tell us there was nothing wrong with the computer at all.
After we picked it up, eventually, when it started happening again for the third or fourth time, we realised the problem:
The "large" (a whopping 26") CRT TV we'd recently started placing it under when not in use caused it... A few days away from the TV to dishcharge it, and it was fine - hence why the repair technician didn't find anything.
I know this story is a bit unrelated, but your story brought it to mind (probably the keywords child, repaired several times, etc): When I was a kid my father had a Nokia 3310. Any early Nokia owner would remember how you could configure your own 'welcome message', which would flash up on the phone's monochrome screen for a second while it boots. My Dad was an engineer, and would get totally hung up on the minutiae of anything technical. Picture him spending a totally inappropriate amount of time adjusting the TV's audio settings, for instance. One day I thought I'd play a prank on him, and changed the welcome message on his phone to say 'SIM CARD ERROR'. It sent him completely bananas, and triggered this huge quest to remedy the problem. He ended up taking it back to the local Vodafone store where he bought it multiple times, and they couldn't figure out what was wrong either. Maybe they got the joke and thought it was too funny, who knows. The store ended up escalating the case to Nokia and authorised an RMA. At which point I figured the joke had gone too far, and I reset the welcome message to 'Dad's Phone'. For some reason, he was happy just to leave it at that. I never confessed what I'd done! It caused way too much fuss, and I'd have been absolutely in for it if anyone figured it out!
Each time it took several days before they repair centre got to it, and they then contacted us to tell us there was nothing wrong with the computer at all.
After we picked it up, eventually, when it started happening again for the third or fourth time, we realised the problem:
The "large" (a whopping 26") CRT TV we'd recently started placing it under when not in use caused it... A few days away from the TV to dishcharge it, and it was fine - hence why the repair technician didn't find anything.