Despite all the vendors of 802.11 (b & g) claiming that their kit is 100% compatible, I have found a few instances where one wifi network card will just not, ever, connect to a given WAP, while other brands have no problems.
This can even (in one instance) be a problem between a wifi NIC and a WAP from the SAME vendor (Netgear)!
But in all the cases I have come across it has been NO connection, not periodic failure.
Good advice so far, to which I would add - ensure that you don't have both Windows AND the card vendor's config software trying to manage the wifi card at the same time.
SD