Autosave happens automatically when you arrive in Solstheim and Windhelm docks. Sames goes when you read any of Hermaeus Mora's book and you get transported to his realm and when you read them to go back to Solstheim.
As a fellow PS3 gamer, here is what I do to avoid the game freezing:
1. When I arrive in Solstheim, I let the autosave finish by letting my character idle. No entering houses and no dialogues. My game crashed when I entered the temple.
2. Save first before reading the black books. When you read the black books of Hermaeus Mora, stay idle and don't jump into the action right away. Combat has a tendency of freezing your game when the autosave has yet to finish.
3. Occasionally DELETE autosaves in the PS3's XMB scrren after you save your game. Accumulated saves slows down the game to the point of freezing.
4. Never attempt to save while the game is autosaving.
5. Finally, here is the bug that I have had much trouble: Overwriting an existing save file. Imagine my frustration when the game freezes when the save progress is at around 99%. So I make sure I save a new game file and never overwrite.
I hope those helps....
