When the Wi-Fi module dies, or rather drops its connection, does the motors or servos continue to run? When the connection is dropped, the NXT will repeat the block the code it currently is in (only in RobotC, I don't think it happens in LabView).
If this happens and you aren't running a program, then 1) your external battery may not have enough power, or 2) the router the module is trying to connect is bad, too far away, or there is too much interference present, like computers connected to the Internet thru the router. Finally make sure your module is connected correctly, and update your FCS to version 2.0 if you haven't. Anyway, for some reason whenever my team tests out the FCS the module would always drop the connection, but in the competition it work fine, except for a couple occasions.
http://usfirst.org/uploadedFiles/Com..._Practices.pdf