wireless access listen in iwlist scan but nothing in iwconfig

I have been using my laptop ( Centrino 2200 wireless with ipw2200) in a
variety of places and have noticed that a number of times I have been
getting access points listed in iwlist scan, but nothing comes up in
iwconfig-- the card is not associating with any of the access points listed
by iwlist. Sometimes also halfway through a session it will drop the access
point (iwconfig contains nothing but iwlist still shows the access points)
but if I modprobe -r ipw2200 (version .17) and then modprobe ipw22900
again, it is again connected.