came across this, it seems it could be 60GB HDD users mostly affected, but uninstalling and reinstalling the game is supposed to fix it.
I have a 40GB version and no freezes or crashes, yet
GTA4 (PS3) Freezing Issue; Fix Suggested | Game Stooge
GTA IV freezes sometimes, but there's a fix - PS3 Fanboy
Will give it a proper test tonight and every night for the next ages