Have you tried a factory reset? You should also consider the other pc settings as well, it may not be the SHUB 2. For example, have you updated the drivers, what link speed are they connecting at?. How far away are the other PC's, what is the signal strength? What version of windows, is it fully patched? Have you tried going to the manufacturer of the wifi cards instead of the PC maker as generally I find that companies such as Dell have outdated drivers.
Here is a good place for drivers:
http://www.station-drivers.com
Get Netstumbler for testing wifi
http://www.netstumbler.com