The object flips after 180 degree of rotation - c++

Question

I m new to OpenGL. I have a viewer for a point cloud.
When I rotate the point cloud around x-axis or z-axis , it gets flipped or inverted after 180 degrees and flips back again at 0 degrees.
From 0-180 degrees it works fine and from 180-360 degrees it flips.
The following is a code snippet for the paint event
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
m_center[0] = (m_minX+m_maxX)/2.0f;
m_center[1] = (m_minY+m_maxY)/2.0f;
m_center[2] = (m_minZ+m_maxZ)/2.0f;
m_radius = static_cast<float>(std::sqrt((m_maxX-m_center[0])*(m_maxX-m_center[0])
+(m_maxY-m_center[1])*(m_maxY-m_center[1])+(m_maxZ-m_center[2])*(m_maxZ-m_center[2])));
m_fDistance = m_radius/0.57735f; //where 0.57735f is tan(30 degrees)
m_dNear = m_fDistance - 3*m_radius;
m_dFar = m_fDistance + 3*m_radius;
glLoadIdentity();
glPushMatrix();
// glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-m_zoomFactor*2*m_radius, +m_zoomFactor*2*m_radius, -m_zoomFactor*2*m_radius, +m_zoomFactor*2*m_radius, m_dNear, m_dFar);
glMatrixMode(GL_MODELVIEW);
glRotatef(m_modelRotation.x/16.0, 1.0, 0.0, 0.0);
glRotatef(m_modelRotation.y/16.0, 0.0, 1.0, 0.0);
glRotatef(m_modelRotation.z/16.0, 0.0, 0.0, 1.0);
glScalef(m_modelScale.x,m_modelScale.y,m_modelScale.z);
switch(m_shapeMode)
{
case eShapePointCloud:
drawPoints();
break;
case eShapeSolid:
drawQuads();
break;
default:
qDebug()<<"No shape is specified, so use the points shape.";
drawPoints();
break;
}
glPopMatrix();
Here is the code for the mouse move event
void OglWidget::mouseMoveEvent(QMouseEvent *event)
{
GLfloat dx = GLfloat(event->x() - m_lastPos.x()) ;/// width();
GLfloat dy = GLfloat(event->y() - m_lastPos.y()) ;/// height();
if (event->buttons() & Qt::LeftButton)
{
setXRotation(m_modelRotation.x + 8*dy);
setZRotation(m_modelRotation.z + 8*dx);
this->setPropertyValue(1,m_modelRotation);
}
m_lastPos = event->pos();
}