After a few hours no tv picture with MediaPortal - Why? (1 Viewer)

[BeTZe]

MediaPortal Version: 1.1.0
MediaPortal Skin: StreamMP
Windows Version: Windows XP SP3
CPU Type: Intel Atom 230, 1,6 GHz
HDD: Samsung 160GB
Memory: 2GB
Motherboard:
Video Card:
Video Card Driver:
Sound Card:
Sound Card AC3:
Sound Card Driver:
1. TV Card:
1. TV Card Type:
1. TV Card Driver:
2. TV Card:
2. TV Card Type:
2. TV Card Driver:
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec:
MPEG2 Audio Codec:
h.264 Video Codec:
Satelite/CableTV Provider:
HTPC Case: Acer Revo
Cooling:
Power Supply:
Remote: Microsoft
TV: Samsung
TV - HTPC Connection: HDMI

Hello
sorry for my english. I have a Problem with MediaPortal. MediaPortal runs very good. It starts when i boot Windows XP. When i start the pc and on my tv i have the HDMI channel active all is good. I have on tv my pc desktop and then MediaPortal. I can look videos and listen music.
When i know change the TV Chaennel for 1 or 2 hours or i turn of my tv and go after a time back to the HDMI Channel i get on the tv the message "No Signal". I don't the MediaPortal. The Only thing i can do is to turn off the PC and reboot.

When i start the PC and doesn't start MediaPortal, i have only my PC Desktop, the problem is not. Then i can turn off the TV for Hours and when i turn it on i have the PC Desktop.

Can somebody help me? Why i have no Picture when i have startet MediaPortal? Goes MediaPortal in StandBy?

Thanks for help.