iPod Touch WiFi problems ...

Recently I have updated my iPod Touch 2G from 2.2.1 to 3.1.1. and since I updated it, it comes with this situation:
-It connects to the internet
-The internet stops working after a really little while, but the WiFi signal bars still showing
-I power off the internet of my iPod and turn it on after a while
-Same problem repeats

When I was on the 2.2.1 version I never had this problem and I have already tried many things to make it work, such has unplug my router and plug it in again and such.

So, how can I fix this problem?

Thanks in advance
-Chris S.

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