I've been able to do so, but with 3dsmax7, don't own 8. However, i found out that most of the times it's because some oddities with the model, or the bones. For example, once a model would export ok, but the bones where upside down, thus a really odd animation ocurred.
The problem there was that my model had it's pivot rotated and so when exported it got rotated or whatever. I could solve that on easily, and most times is because of situations like that that it exports oddly.
If you'd like i could have a look at it and check it out. I couldn't explain you how to do it, since most times i have to work out things in some odd ways.
Further on my stuff at...