I'm not your best resource on this topic, but lacking another response ...
Look thru the X11 log "/var/log/X11 and you should see some lines where X negotiates/tests for available
My ati bsed laptop (using the fglrx driver) begins
Quote:
(II) fglrx(0): Total of 12 modes found for primary display.
(--) fglrx(0): Virtual size is 1400x1050 (pitch 0)
(**) fglrx(0): *Mode "1400x1050": 85.0 MHz (scaled from 0.0 MHz), 53.1 kHz, 50.0 Hz
(II) fglrx(0): Modeline "1400x1050" 84.96 1400 1472 1512 1600 1050 1052 1053 1062
(**) fglrx(0): *Mode "1280x1024": 85.0 MHz (scaled from 0.0 MHz), 53.1 kHz, 50.0 Hz
(II) fglrx(0): Modeline "1280x1024" 84.96 1280 1408 1448 1600 1024 1039 1040 1062
(**) fglrx(0): *Mode "1152x864": 85.0 MHz (scaled from 0.0 MHz), 53.1 kHz, 50.0 Hz
(II) fglrx(0): Modeline "1152x864" 84.96 1152 1344 1384 1600 864 959 960 1062
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and so on for a couple dozen lines.
You may see a note on 1280x1024 that indicates the issue - frequency out of range or whatever. IN that case you'll need to add a "Modes" line to the /etc/X11/xorg.conf. There are many examples of doing this (use the search tab on this forum).