Video driver seems to ignore modeline

[phong at phong.org]

I've got a Dell 2001FP LCD flat panel (native resolution 1600x1200) hooked up to
a Radeon 7000 series card via DVI (lspci sez "0000:01:00.0 VGA compatible
controller: ATI Technologies Inc Radeon RV100 QY [Radeon 7000/VE]").  After X
has been started for a while (random amount of time), the image shifts to the
right about half an inch, leaving a black space on the left and part of the
image cut off on the right.  The problem only happens in Linux - never in
Windows.  I investigated what timings were used by the Windows driver and found
that they were indeed different than what the X driver was using.  So I added
the following modeline, which is identical to the Windows timings, to the
Monitor section of my xorg.conf:

ModeLine "1600x1200-Dell"  162.00  1600 1672 1864 2160  1200 1201 1204 1250

The problem is, when I start X, it seems to ignore that modeline and do its own
thing.  Snippet from the log:

-------------------------------------------------------------
...
(II) RADEON(0): DFP table revision: 3
(II) RADEON(0): Panel infos found from DDC detailed: 1600x1200
(II) RADEON(0): Valid Mode from Detailed timing table: 1600x1200
(II) RADEON(0): Valid Mode from standard timing table: 1600x1200-Dell
(II) RADEON(0): Total of 2 mode(s) found.
(II) RADEON(0): Total number of valid DDC mode(s) found: 2
(II) RADEON(0): Valid mode using on-chip RMX: 1600x1200-Dell
(II) RADEON(0): Valid mode using on-chip RMX: 1400x1050
(II) RADEON(0): Valid mode using on-chip RMX: 1280x1024
(II) RADEON(0): Valid mode using on-chip RMX: 1152x864
(II) RADEON(0): Valid mode using on-chip RMX: 1024x768
(II) RADEON(0): Valid mode using on-chip RMX: 800x600
(II) RADEON(0): Valid mode using on-chip RMX: 640x480
(II) RADEON(0): Total number of valid FP mode(s) found: 7
(--) RADEON(0): Virtual size is 1600x1200 (pitch 1600)
(**) RADEON(0): *Mode "1600x1200-Dell": 162.0 MHz (scaled from 0.0 MHz), 75.0
kHz, 60.0 Hz
(II) RADEON(0): Modeline "1600x1200-Dell"  162.00  1600 1664 1856 2160  1200
1201 1204 1250
(**) RADEON(0): *Mode "1400x1050": 162.0 MHz (scaled from 0.0 MHz), 75.0 kHz,
60.0 Hz
(II) RADEON(0): Modeline "1400x1050"  162.00  1400 1664 1856 2160  1050 1201
1204 1250
...
-------------------------------------------------------------

I've tried both the "radeon" and "ati" drivers with the same effect.  I was able
to confirm that my modeline fixes the problem by using xvidtune to tweak the
timings after X has started.

Is there any way to get it to honor my modeline?