was simply dust.
Today I "repaired" another laptop computer. It had difficulties remaining online with the wireless network. After some minutes surfing the NetworkManager applet displayed a message "connection lost". Looking into the log files revealed usb disconnect messages from the integrated sis wireless card, which is - why the heck ever - connected via usb instead of minipci. As the problem was reported to be an issue with Windows as well as with the mainly used ubuntu linux I assumed it could be a hardware issue. And opening the laptop revealed a nearly 1cm thick dust layer just between the heat spreader and the fan. Removing the dust seemed to have healed the laptop.
This was the third or fourth time I could reactivate a laptop computer simply by removing dust. Maybe the manufacturers should pay more attention to this fact and should try to build the devices in a way that the dust could not that easy enter it?