Okay, I'll quote myself from another thread:
"I've noticed that about 5 or 10 minutes after I put my vox to sleep, it briefly wakes up. Once that happens, it is DOA and needs to be hard reset. The last time it did this, I unlocked it as it was briefly awake. There was an exclamation mark in the status bar with the message "SD Card Unexpectedly Removed". Of course, nothing worked at that point and it needed a hard reset. Is the internal sd card getting unmounted at some point in the sleep process? "
So in my experience with wifi off, the wake-up is the trigger for the sleep death. I maintain the two issues are related.