I had a similar problem when I first bought my iPronto. It was fixed by replacing the WiFi card. It was a known problem at the time, and Philips sent new cards out to retailers. Try taking it up with whoever you bought it from. I bought mine from Letsautomate, and they sent me a free card. Fixed my problem, although I was led to believe that later firmware did not suffer from this one.

I had exactly the same symptoms. Very annoying to have to remove the battery to get it to reboot.