I finally got my 2004 iPod working again after a guy at the Seattle Goodwill showed me the FireWire trick
He grabbed a $3 FireWire cable from the bin and said my USB-only charging was why it kept dying at 40%. Has anyone else had an old iPod that only revives on FireWire, or is it just the 3rd gen models?
The FireWire thing isn't just a 3rd gen issue, pretty much any iPod with a dock connector from that era will charge better on FireWire because USB just didn't push enough power back then. USB charging got way better on later models but the older ones really do need that FireWire boost to hit 100%.