Oups, forgot to respond to that. Yes, that could be the issue, I'm still not able to reproduce it though =/. I'm glad you managed to get it working anyway!
Also, big updates on the software. The act animations are now being fully rendered; this has been on the todo list for weeks (if not months). I've added a ton of other stuff you won't notice, but the program is really, really stable now.
There should no more UI lag
Any threaded operation can be cancelled
Added a third layer on the GRF manipulation, to ensure your data will not get corrupted / bugged / lost
Using a new engine for the image processing (should be faster to load data, it uses its own structure and the images will never be loaded unless they're being used)
Upgraded these formats : act, spr, gnd, grf, rsm, rsw, pal and rgz.
All windows now use a canvas (easier to modify / make new windows)
Preview images can be saved as Indexed8 bmp images, useful to generate minimaps
Changed the location of the files so that they use the temp folder instead of throwing everything in the application's folder (it was getting messy)
The speed of extraction and packing shouldn't have changed at all even with the new features
I've added the source code of the application for those interested (http://www.mediafire.com/download.php?7z6hkdag4ayj8rs). If you have questions feel free to ask, but the sub project GRFTester shows pretty much everything you would need to know in order to make your own GRF application. You need these dependencies for a new project : GRF.dll, ErrorManager.dll, Utilities.dll and Ionic.Zlib.dll. The library can read almost any file format and I believe that should be helpful for making applications! Everything's in C# so that should be easy to understand and customize.
If you have an idea to improve the software or simply a new feature you'd like to see, go ahead and ask!