Display blank when TV turned on

Hi all

My new HTPC has onboard Radeon 3000 graphics which is connected to my Panasonic plasma TV via a DVI to HDMI cable. The problem is, with the HTPC on, I turn on the TV but the display remains blank. I can hear the Windows Media Centre menu when I press buttons on the remote but I can't see anything. The only way to get the picture back is to put the HTPC to sleep, then wake it up again. Obviously, this would be no good if the HTPC was recording TV, so how do I get it so that the display comes on as normal when the HTPC is already switched on?

Many thanks

Michael
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