Well here is what I know from my experience with WPA
1 it decreases the signal strength so machines that would connect fine with wep/no security could possibly have an issue with wpa
2 the key refresh rate seems to play a part in when people lose connection, the shorter the interval, the more I saw cards reconnecting
3 using the built in software vs the wireless zero connector on the computers would give me different results too. If I did wireless zero I could always expect the same thing from every machine so if one worked, I could bet they all would. if I used the dell connect manager or aetheros or whatever came with the card, it would be a different result everytime I setup the connection.
Hope that helps.