I'm developing a suspicion that something in the hardware is marginal. Maybe the eMMC chips, maybe something else. I've seen a few phones on eBay being sold because there are "stuck". I've even bought a couple and reflashed them. Since the sellers seemed uninformed about reflashing, I imagine they were just using the phones normally instead of messing around.
On one of my MT4GS, it lost its marbles twice while I was actually using it (and doing nothing special). It got really slow and jerky, and then it eventually shutdown. No signs of life except a single buzz if I long-press the power button.
After this happened the first time, I tried it again in a couple days and was able to get into hboot. I did a nandroid restore. Everything was fine for 4-5 days when it repeated its contrariness. (That was yesterday; it's currently on the shelf with me waiting for it to come to its senses.)
Luckily, I had another idle MT4GS and restored it with my CWM nandroid backup, so I'm back on the air. That first phone has me wondering, though.