Rotation Error on Object at 90° and -90° - ios

Question

I'm working on an app which allows the user to rotate an object with iOS touch controls.
I have the following script working fine, with 1 issue that I can't seem to crack.
GameObject mainCamera;
public Camera camMain;
// One Touch Rotation
public float rotateSpeed = 0.5f;
static float pitch = 0.0f, yaw = 0.0f, zed = 0.0f, pitchBravo = 0.0f, yawBravo = 0.0f;
// Two Touch Zoom
public float perspectiveZoomSpeed = 0.1f;
// Three Touch Pan
public float panSpeed = 0.5f;
private float xAxis = 0.0f, yAxis = 0.0f;
private float xMain, yMain, zMain;
// Game Objects, Public or Private
private GameObject bravo;
void Update()
{
// Grabs Bravo
bravo = GameObject.Find ("bravo");
pitch = bravo.transform.eulerAngles.x;
yaw = bravo.transform.eulerAngles.y;
// One Touch controls rotation of Bravo
if (Input.touchCount == 1)
{
// Retrieves a single touch and names it TouchZero
Touch touchZero = Input.GetTouch (0);
// The start of the rotation will be aligned with Bravo's current rotation
//pitch = bravo.transform.eulerAngles.x;
//yaw = bravo.transform.eulerAngles.y;
// Times the difference in position of touch between frames by the rotation speed. deltaTime to keep movement consistent on all devices
pitch += touchZero.deltaPosition.y * rotateSpeed * Time.deltaTime;
yaw -= touchZero.deltaPosition.x * rotateSpeed * Time.deltaTime;
// Assigns the new eulerAngles to Bravo
bravo.transform.eulerAngles = new Vector3 (pitch, yaw, 0.0f);
}
// Two Touch contols the Field of View of the Camera aka. Zoom
if (Input.touchCount == 2)
{
mainCamera = GameObject.Find("main");
// Store both touches.
Touch touchZero = Input.GetTouch(0);
Touch touchOne = Input.GetTouch(1);
// Find the position in the previous frame of each touch.
Vector2 touchZeroPrevPos = touchZero.position - touchZero.deltaPosition;
Vector2 touchOnePrevPos = touchOne.position - touchOne.deltaPosition;
// Find the magnitude of the vector (the distance) between the touches in each frame.
float prevTouchDeltaMag = (touchZeroPrevPos - touchOnePrevPos).magnitude;
float touchDeltaMag = (touchZero.position - touchOne.position).magnitude;
// Find the difference in the distances between each frame.
float deltaMagnitudeDiff = prevTouchDeltaMag - touchDeltaMag;
// Otherwise change the field of view based on the change in distance between the touches.
camMain.fieldOfView += deltaMagnitudeDiff * perspectiveZoomSpeed;
// Clamp the field of view to make sure it's between 0 and 180.
camMain.fieldOfView = Mathf.Clamp(camMain.fieldOfView, 2.0f, 30.0f);
xAxis -= touchOne.deltaPosition.x * panSpeed * Time.deltaTime;
yAxis -= touchOne.deltaPosition.y * panSpeed * Time.deltaTime;
zMain = mainCamera.transform.position.z;
mainCamera.transform.position = new Vector3 (xAxis, yAxis, zMain);
}
}
With this script the object rotates left and right perfectly, 360°. But when the model is rotated up and down it gets to the 90° or -90° mark and bugs out, not allowing the user to carry on rotating.
I've done a bit of research and I believe it may have something to do with gimbal lock, my lack of knowledge on the subject means I haven't been able to come up with a fix.
Any help is appreciated.
GIF of bugged rotation