Proper Camera Rotation

In order to have a better look at the weird artefacts on my terrain, I tried to add a mouse & keyboard controlled camera. Matrix.RotationYawPitchRoll doesn't seem to work for me - if I rotate my camera 90 degrees about the Y axis (yaw) looking to my left then try to rotate about the Z axis (roll) so the view spins clockwise it actually rotates around the X axis (pitch) and ends up looking up and down.

(I suppose I should've said on the first post that I'm writing the game in Managed DirectX with C#).

Anyways, I had this problem before when I was good at geometry so I was sure I could do it again. Unfortunately years of boozing have destroyed any remnants of my secondary school geometry and the results of my sin/cos calculations were that I should go get my old computer out of the shed and get the old code. Here it is if anyone has the same problem:

private void Rotate(float yaw, float pitch, float roll)
{
    // We'll create 3 matrices for the combined rotation
    Matrix yawMatrix = new Matrix ();
    Matrix pitchMatrix = new Matrix ();
    Matrix rollMatrix = new Matrix ();

    _localMatrix.Translate(_position);

    // Reset it's orientation vectors to make sure they're all
    // sqare against each other and unit length
    _look.Normalize();
    _right = Vector3.Cross(_up, _look);
    _right.Normalize();
    _up = Vector3.Cross(_look, _right);
    _up.Normalize();

    // The pitch rotation will be done about the x axis (right)
    pitchMatrix.RotateAxis(_right, pitch);
    // The yaw rotation will be done about the y axis (up)
    yawMatrix.RotateAxis(_up, yaw);
    // The roll rotation will be done about the z axis (look)
    rollMatrix.RotateAxis(_look, roll);

    // Rotate the local axis about these matrices so we can tell
    // how the object is now oriented after rotation

    // Rotate the _look and _right about the yaw rotation
    _look.TransformCoordinate(yawMatrix);
    _right.TransformCoordinate(yawMatrix);

    // Rotate the _up and _right about the roll rotation
    _up.TransformCoordinate(rollMatrix);
    _right.TransformCoordinate(rollMatrix);

    // Rotate the _look and _up about the pitch rotation
    _look.TransformCoordinate(pitchMatrix);
    _up.TransformCoordinate(pitchMatrix);

    // Store this in the local matrix for translation when rendering
    _localMatrix.M11 = _right.X;
    _localMatrix.M12 = _right.Y;
    _localMatrix.M13 = _right.Z;

    _localMatrix.M21 = _up.X;
    _localMatrix.M22 = _up.Y;
    _localMatrix.M23 = _up.Z;

    _localMatrix.M31 = _look.X;
    _localMatrix.M32 = _look.Y;
    _localMatrix.M33 = _look.Z;
}