Be sure to check the wifi trouble shooting sticky. What I think is happening is that your wifi hangs due to the many possibilities talked about in the wifi thread. Wifi and 3G data do not run at the same time, this is normal, wifi takes priority. Since yours is in a frozen state android doesn't know to turn back on 3G and route data through that connection. Toggle wifi off and ot will go through 3G. Toggle it back on and wifi will work again until it freezes again.
Leaked 2.2 fixes alot but not all, it is hoped that the official 2.2 will have it fixed completely. It better considering how late it is already.
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