What steps will reproduce the problem?
1. Movement from rest or flight take-off/landing
2. casting
3. walking or fighting

What is the expected output? What do you see instead?
I expect the game to function as normal; in fact, it did, until the latest patch was loaded from Blizzard, last week. I generally get 60-70 fps with no hang-ups or freezing using the "good" settings for interface at 1440x900 p resolution. Now, I'm getting 1-7 fps when QH2 is enabled; the game interface sometimes crashes, my toon and game camera start spinning like a top at random times, and in general the game becomes impossible to play - one time, my computer locked up. If I disable QH2, all the issues stop.

What version of the product are you using? 0.19

Do you have an error log of what happened? No, QH2 doesn't give one. I do get messages: 'game interface failed because of add-on', just before the madness begins again. At other times, I get random messages about undefined destinations or undefined quest objectives - those stopped when I disabled the add-on DBs.

Please provide any additional information below.

It's a very strange issue, I know, and has only been happening for six days, now. I've removed and reinstalled the interface, disabled all the databases, removed and reinstalled QH2, after reinstalling the game - it makes no difference. When I removed QH2 and run the game, I don't have problems. If I play my high-level toons in classic or BC game areas with QH2, I don't have problems. If I run in WoLK areas, the issue is mild, but it becomes nearly impossible playing in Cataclysm areas and Firelands. I'm running an odd OS, as far as things go: Win XP x64 Pro; AMD phenom II processor, nVidia GT 240 card.