I had this happen to me a couple of times. It seems that there may be a bug with the Monoliths saving experience/Level stats. However, it does seem to save any items you have picked up or used, like potions and it also saves your location. It is odd, like it does a partial save of your location, but nothing else.
I had issues with the game locking under the following circumstances.
1. Too many active side quests.
2. Having people follow you around from unfinished side quests.
I am leaning more towards #1 being the primary cause than #2.
My experience:
Silver - activated very few side quests, didn't freeze much at all
Gold - Activated quite a few side quests, but didnt complete many of them - froze quite a bit
Platinum - Activated 1-2 side quests at a time, and completed them before activating more - froze a little, but not too much
Niob - Activated no side quests unless I intended to finish them right away - rarely any freezes - maybe 1/50th of what I had on Gold.
My system rarely freezes during free world play, I had most of the issues during campaign. It does freeze on occasion, but it is very rare in free world.
Disable - Gore and Buff Effects - speeds up play a little, and keeps the system from having to display these constant effects.
I never unplug my PS3, there is a power switch on the back of it similar to a computer. At least on my PS3 there is.