Thank you MrBigzy, I have weighted the joints properly, it's just i'm not so sure about the terminologies...
Anyways, I think i've found the problem, I have naively thought that animation was just moving a few joints, then setting some keys, but as you said, I must use the graph editor.. Also, I have read a page, and I think I know my problem
Quote:Euler-angled rotation is prone to artifacts such as gimbal lock and flipping. Gimbal lock occurs when rotations about a single axis cause unwanted rotations about complementary axes or when axes become coincident. Flipping occurs when angles unexpectedly wrap around positive or negative 180 degrees during Euler-angled rotation interpolation between keyframes.
So thanks again for all the help. I think graph editor is the solution
Edit: It's fixed! I applied a filter in the graph editor, and now the rotation is much better. Thanks very much!