Support Data Connections goes into standby

Evening all,

I have a HTC desire on three. I use the phone at work to provide me a data connection for doing some light intensity data stuff. I tether the phone via USB. I notice that after about 30 mins or so I loose connectivity. When I pick the phone up and unlock it I notice there is no HSDPA icon but i do have a full bar of signal. It then appears after a few seconds to wake up the data connection after I prompt it do by say refreshing the weather app or facebook. Has anyone any experience of this? I'd like to force the connection open if possible. To my knowledge I haven't installed any power saving apps or such.

am running android 2.2

Thanks,

Tibzzz