Its been a while since i used EvoX as dashboard . . . maybe the resoltuion needs to be set in the original MS dashboard. Just a thought
XBMP can be used as a dashboard it has the ability to set the resolution by the config.xml . . .
<!-- Set the resolution you want XBMP to run in -->
<!-- 1 - Use Xbox video settings to determing -->
<!-- 2 - NTSC 720x480 @ 4:3 -->
<!-- 3 - NTSC 720x480 @ 16:9 -->
<!-- 4 - PAL 720x576 @ 4:3 -->
<!-- 5 - PAL 720x576 @ 16:9 -->
<!-- 6 - HDTV 1280x720 @ 16:9 -->
<!-- 7 - HDTV 1920x1080 @ 16:9 -->
<resolution>1</resolution>