Most of the time, this is caused by a low signal strength from the wireless access points. Some adapters will be able to pick up on weaker signals, while others will drop the connection sporadically. If you are using an adapter that works solely on the 2.4 GHz frequencies (such as B and G cards), then try not to operate electronics that also work on the frequency (such as 2.4 GHz phones and microwaves) in the vicinity.