QQuaternion how to rotate only x and y??

[QT-static-prgm]

Hi,

i want to rotate only in x and y direction, but it does not work that good.

float pitch, yaw, roll;
m_rotation.getEulerAngles(&pitch, &yaw, &roll);
pitch = (diff.y() * 0.5 + pitch);
yaw = (diff.x() * 0.5 + yaw);
std::cout << pitch << " - " << yaw << std::endl;
m_rotation = QQuaternion::fromEulerAngles(pitch, yaw, roll);

yaw works fine, but pitch does not. it is limited to [-90,90]. But if i move the mouse not only up/down, but a little sideway, it moves over that limit but does crazy things like jumping somewhere. And most times i get nan from pitch and yaw and the model is not displayed anymore.

Any ideas why this happens, or how to rotate only in x/y direction using Quaternion??