Dave,
I have an interesting experience regarding the reset issue.
I dont't use any Amazon case with my Kindle 3 but the resets in sleep mode occured on regular basis (especially when I've travelled). I opened my kindle and I've noticed that there is a small gap (< 1mm) between battery and display so the battery could slightly jump up and down (only the upper end of battery is mounted by screws). So I've fixed bottom end of battery by thin piece of soft foam, I've put it between battery and display.
The random resets absolutely disappeared, the kindle works already 3 weeks fully stable. I don't understand the cause because battery related contacts are plated by gold of course. Maybe battery power circuits are deficiently stabilized by capacitors.