1) Check that you installed the latest drivers
viewtopic.php?f=53&t=17822) Disable display 1 (LVDS, not connected) - then you should have all supported resolutions on display 2 (SDVO).
viewtopic.php?f=53&t=15243) Check that LCD clock is enabled in BIOS (I remember we did enable it before we sold you the computer but please check anyway). If it's enabled, then maybe disabling it will help.