Interferance?

As most of the Wlan equipment work in the 2.4GHz band.. interfering equipment could include..

Microwave Oven (lovely source of wideband RF in the 2.4Ghz band)
Some Cordless phones (some are in the same Freq assignment as the WLAN system)
Some Mobile phones (third harmonic of the mobile phone assignment, in some countries, land in the midle of the WLAN assignment.. and while the power level may drop the noise bandwidth increases so more WLAN channels are affected)
Security systems, Radar motion detectors..

Get a hold of Netstumbler and give it a run.. monitor the AP/router for a while .. see how the signals fare especialy when the netwok drops..

Cheers

BTW; I am by no means an expert with WLAN.. But 20years as a radio tech wasn't for nothing.. I will conceed that there are members here that would be far more familure with WLAN probs than I..