Question Desire Wi-Fi annoyance

Hi friends

I have a 2 month old HTC desire. it is not rooted ans is running on 2.2

Since the time i've bought the phone i have a problem with wi-fi. once i put my phone to normal sleep (short press power button) when i again unlock my screen after sometime my wi-fi is still connected but the problem is that i cant load any website or anything else through wi-fi unless i turn it off and on again.

Can someone help me on this?

thanks