Well, for the past two days, my NAT type was moderate.
The game was running smoothly with no problems at all. However, while I was playing now, an update popped up (regarding the playlist). The NAT type returned to OPEN, as soon as I joined a game, the ps3 froze the first couple of seconds.
Assuming that being a host will cause the hard freeze. In addition to the trophy synchronization while pressing select to view the scoreboard in-game.
To sum it up, keep your NAT type moderate in order to reduce the hard freeze issue.
Best of luck.
Me, as things occurred during whilst playing.
mine has always been on moderate an i get them all the time so it can't be that.