Maybe the battery. If it's draining down during the day, that might be why it's not doing anything early in the day, but does so later.
Another thing is on earlier OS versions, the 8320 seemed to have some problems with non-paired bluetooth devices in the area. It seemed like it would keep trying to communicate anyway, which would drain the battery. I haven't tested since going to 4.5, but maybe it still does that.
I think I'm starting to have problems with my 2 year old 8320 battery. It has rebooted while running slingplayer for just a little while, and the battery shows 2/3 full when it comes back up. I can run that while on the car charger for hours without problems. That's a high drain app so I think the battery is getting too old to deal with that drain.