Regarding these lock-ups... Around 5 days ago or so, my lock-ups and DDE's were starting to happen quite a bit, like once every 2 hours of play. I changed two things one night, and since then I've had only 1 lock-up and 0 DDE's. Here's what I did:
1) Deleted all but 5 save game files.
2) Stopped carrying so much shit around. Seriously, I had a TON of potions and spell scrolls on me, so I went to a appropriate merchant and sold off almost everything I had on me, and have since kept my inventory managable.
Since I did those two things, very very few lock-ups. Now, the question is, did doing those two things fix the problem? Who knows, could have been anything. I never moved my 360, so ventilation was the same (besides, I've never had another crash for as long as I've owned my 360, except once in PGR3 oddly enough). I tried the clear cache thing soon after the game was released, tried it a couple times actually, but it never helped my crashes/dirty disc errors.
This is just fyi, ymmv.