Honestly not sure what was going on here. I kept having full-blown crash issues -- and when I say crash, I mean 'takes the computer with it'. Full on black screen, have to reboot. There may have been issues between the game and the graphics card (an nVidia 9600GT), but this is a recent development.
So. Updated drivers, reinstalled .NET 1.1, no change. Then I uninstalled DDO and prepped to reinstall, and found out that ZoneAlarm was getting in the way. Hmm. Wonder if that's affecting things? Uninstalled ZA and replaced it with OnlineArmor; we'll see what that does.
Interestingly, most of the crashes were in the quest 'Where There's Smoke' -- two coming when I'd turned the valve, another triggered when I tried to hit the orcs at the barricade with a Meteor Swarm cookie. If there's a pattern, it's beyond me.