I am having the same lag issues when outside in Big MT as a lot are.
I seem to notice it happening the most when enemies approach or rez. And sometimes when leaving a building into the "overworld" area.
I turned off "save on travel" and I play while not logged into the PSN to use less of the system RAM, but it seems like it is a RAM issue...... mainly, not enough.
The reason why i suspect it is the RAM is the XMB is bogged down when this happens.
Can this possibly be fixed by lowering the draw distance 9or something) in the Big MT outside area?
Or is there a way to refresh the RAM of what is loaded without turning off the game and reloading?
Oh, I am playing on a save file that is 12mb and was created after patch 1.01 with the classic pack DLC. I also have Dead Money installed and Honest Hearts.