I have even more hours on one of my characters, but have only had lag a few times, and each time it was during a crazy battle with lots of spells blasting everywhere.
This seems to help for me though....
- Do a hard save once and awhile (usually at the end of a play-session) don't just rely on auto saves when you reboot your game the next day.
- After extensive playing, turn off the Xbox for a minute. Make a sandwhich, reboot.
- Clear the System Cache occasionally (this doesn't delete any save files, you just will be prompted to download any patches again)
Seems to help, but like I said, I've had very little problems to begin with.