iphone 3g black screen: iPhone Wifi Connection Does Not Work ...

Hi,

My iPhone seems to barely pick up any wifi connections when I search for networks.
When it does find my connection it connects but after a few seconds the wifi strength bar goes low and then goes, reverting back to the 3G connection... any advice?

Have tried it on several networks and my flatmates iPhone is fine on our network so I don't believe it is a network issue,

Cheers,

Ollie