Mmmm... not an easy one :
Could be that your AP disconnects because it is perturbated by another AP, which echoes around and have your machines considering connection not valid anymore. Could be also configuration problem, like for XP, when you use WEP encryption, connection is stopped after a while (because machines have an idle timeout set). Check this encryption prob, try also to Uncheck the 'Closed Wireless Network' option...
It works on many networks, the prob you describe is not harware one, must be a config prob...
hth
J