What brand/model of router are you using? Do you have the SSID hidden (or off)?
General stuff you could check if you have not done so already:
-ensure you have enough DCHP leases
-ensure your lease time isn't too long or short
-enable SSID broadcast (people hide them and it does nothing all for security)
-try not connecting to a VAP (virtual access point) if you have been using one.
As a beta feature (and how does this differ from the rest of the firmware?
) the browser can be a bit funny at times, and Kobo officially does not support it, however you could send them a detailed explanation of the problems you have had. I have observed that some web pages (high in flash, java and other components) can cause issues, either extreme lagging or even locking up the reader, however not with 2.6.2 or 2.8.1 as of yet.