honestly speaking. It's too easy (in this whole thread) to look only at other equipment (router etc) and not at the NP3300.
The router is a Swisscom Centro grande (Motorola). Distance to the NP3300 = 5m. No walls. Wifi configured according to the hints in this forum (fixed IP, non-crowded channel, etc). I tried to rotate the NP3300 as well as the router. Doesn't matter what I do: the NP3300 has a very volatile poor connection. E.g. no movements in the room, nothing changes, within 1 minute, the signal strength at the NP3300 varies between complete lost of connection to 3 bars. An iPad, an iPhone, a laptop, another wifi radio have full signal strength when I put them at the same place as the NP3300. To answer the next question: yes the poor connection stays the same even when I switch off all other wifi devices than the NP3300.
I'm really anoyed about this device. It's only the NP3300 which has this heavy connection problems.