Upgraded X to 4.1: Lost 1280x1024 mode

[FGL <walkarg@wco.com>]

I've just upgraded from potato to woody, including X from 3.3 to 4.1. As a
result, I've lost the 1280x1024 mode that I've been using for years. 

I have an ATI Graphics Pro Turbo card (Mach64 chip). In its printed manual,
a 1280x1024 mode is documented with a refresh rate of 60Hz, a horizontal
frequency of 63.9kHz and a pixel clock of 110MHz.

In X 3.3, the output of startx would say

(--) Mach64: PCI: Mach64 GX rev 1, Aperture @ 0xf0000000, Sparse I/O @ 0x02ec
(--) Mach64: Card type: PCI
(--) Mach64: Memory type: 6
(--) Mach64: Clock type: ATI18818-1/ICS2595
(--) Mach64: Maximum allowed dot-clock: 135.000 MHz   
(**) Mach64: Mode "1280x1024": mode clock = 110.000

In X 4.1, it says

(II) ATI(0): Clock range:   9.00 to  80.00 MHz
(II) ATI(0): Not using mode "1280x1024" (bad mode clock/interlace/doublescan)

Does anybody know how can I convince X to use the mode I've been using for
a long time?

FGL