if you look at the cpu usage, you see a huge spike for 1-2 seconds. If you start a stack trace sampling just in that moment, the app hangs in QNetworkAccessCache::timerEvent.

It seems to be related with two different threads using it's own thread specific NAM at the same time.

This is a small extract on the sampling:
---------
1349 QCoreApplication::notifyInternal(QObject*, QEvent*) (in QtCore) + 98
1347 QApplication::notify(QObject*, QEvent*) (in QtGui) + 2132
1345 QApplicationPrivate::notify_helper(QObject*, QEvent*) (in QtGui) + 175
1345 QObject::event(QEvent*) (in QtCore) + 424
1195 QNetworkAccessCache::timerEvent(QTimerEvent*) (in QtNetwork) + 134
1018 QNetworkAccessCache::updateTimer() (in QtNetwork) + 61
512 QDateTime::secsTo(QDateTime const&) const (in QtCore) + 96
412 QDateTimePrivate::getUTC(QDate&, QTime&) const (in QtCore) + 231
---------

And it looks to especially related to Mac OS X, because we were not able to reproduce this on other platforms.