Application freezes for 0.5 sec after keyPressEvent

My application works well on:

Fedora 14 Linux with Qt 4.7.1
Windows Vista with Qt 4.7.0
MacOS with probably also Qt 4.7.x

However, on a Debian Linux system with Qt 4.6.3 I have a strange problem:
The application freezes for about 0.5 seconds after each QWidget::keyPressEvent. Its an 3D graphics application so I have an QTimer that fires a method mainloop every few milliseconds in order to move a camera smoothly. After the keyPressEvent was processed the QTimer fires one or two more times, but then freezes for 0.5 sec. I can "prevent" this problem by moving the mouse the whole time.

It can't easily update to Qt 4.7.x on that system, since I have no root access. However, I could supply Qt 4.7.1 libraries with the application binary. I will try that tomorrow *).

Has anyone similar problems or an idea what could cause this? I searched the bug tracker, but found no related bug that was fixed with Qt 4.7. Maybe it has nothing at all to do with the Qt version.

Best regards,
Coolcat

*) I'm in UTC+2 timezone