The network connection is controlled in sys-net (assuming that you have
kept the default install).
If you open a terminal there you will find it's just vanilla linux as
far as networking goes. You dont say what the wifi card is,but it's
possible that you need to update your drivers or set a specific
configuration.
Remember to do this in the template that sys-net uses.
Also, it's very simple to switch the template and see if you have more
success in maintaining a connection using a Debian template. You can
make this change from the Qubes manager, by looking at and changing
qube settings. (You will need to reboot sys-net to start suing the new
Template.)
Hope this helps
unman