Okay, so I think that all I wrote before was some sort of placebo effect.

The N900 still frequently drops connections. Although it reports that it is connected, no data is transferred, and I have to disconnect and connect again. After a random time (usually between a 10 minutes - 1 hour interval) it just stops working again, although it still says that it is connected.

I tried it with both PSM enabled and disabled. The transmission power is set to max for the AP and the N900 as well.

Another issue is, that both my and my brother's laptop sees the WLAN with max signal strength in the house, but the N900 barely recognizes it.

Is there a possibility that I have a faulty WLAN module?