Setting Orientation of Character in Bullet/Ogre [closed]

Question

I've been suffering from an issue regarding the implementation of orienting characters in a game I'm implementing using Ogre3D and Bullet physics.

What I have: A Direction Vector that the character is moving in, along with its current orientation.

What I need: To set the orientation of the character to face the way it is moving.

I have a snippet of code that sort of does what I want:

btTransform src = body->getCenterOfMassTransform();
btVector3 up = BtOgre::Convert::toBullet(Ogre::Vector3::UNIT_X);
btVector3 normDirection = mDirection.normalized();
btScalar angle = acos(up.dot(normDirection));
btVector3 axis = up.cross(normDirection);
src.setRotation(btQuaternion(axis, angle));
body->setCenterOfMassTransform(src);

Where 'body' is the rigidbody I'm trying to orient.

This snippet has a couple of problems however:

1) When changing direction, it tends to 'jitter' i.e. it rapidly faces one way, then the opposite for a second or so before correcting itself to the orientation it is supposed to be at.

2) Most times that the code is run I get an assertion error from Bullet's btQuaternion on

assert(d != btScalar(0.0));

Can anyone help?

Thanks!