As of Ubuntu 7.10 "Gutsy" and also appearing very early-on in Debian "Lenny" (testing), X now fails to start up for me, whereas X had worked just fine on this same hardware in Debian "Etch" and also in Ubuntu "Edgy" and "Feisty". Seems to be something about Xorg has changed as deployed in the affected releases. Towards the end of my Xorg.0.log there's a whole bunch of lines rejecting every possible resolution (even known-valid ones), like this:

---
(II) imstt(0): Not using default mode "1024x768" (bad mode clock/interlace/doublescan)
(II) imstt(0): Not using default mode "1024x768" (unknown reason)

[Repeat w/ a wide variety of other possible resolutions, all rejected]

(WW) imstt(0): Mode pool is empty
(EE) imstt(0): No valid modes found

Backtrace:
0: /usr/bin/X(xf86SigHandler+0x94) [0x10093f44]
1: [0x100344]
2: /usr/lib/xorg/modules/drivers//imstt_drv.so [0xf941520]
3: /usr/lib/xorg/modules/drivers//imstt_drv.so [0xf94186c]
4: /usr/bin/X(InitOutput+0xb08) [0x1006e2c8]
5: /usr/bin/X(main+0x294) [0x1002e064]
6: /lib/libc.so.6 [0xfc79380]
7: /lib/libc.so.6 [0xfc795c4]

Fatal server error:
Caught signal 11.  Server aborting
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I have tried various mods to my xorg.conf, all to no avail, including:

* Revised HorizSync and VertRefresh to match monitor's actual range;
* Used a tried'n'true custom xorg.conf saved from previous releases;
* Ran 'sudo dpkg-reconfigure xserver-xorg' to build a fresh xorg.conf;
* Add/omit a PCI BusID (I have only one vidcard, and Xorg had worked fine without any BusID in earlier releases);
* Changed: Option "UseFBDev" to "false" in the Device section;
* Added: Option "NoDDC" or "IgnoreEDID" to the Device section;
* Added: Option "Int10Backend" "x86emu" to the ServerLayout section;
* Added a Modeline (actually tried two slightly different modelines produced by different Modeline Generator sites);
* Omit xorg.conf entirely (by renaming).

Interestingly, the "failsafe X" used in in Ubuntu Gutsy, which tries to use the vesa driver, also fails to start X; the Xorg.0.log for that one shows modules being loaded then ends with this:

---
(II) VESA: driver for VESA chipsets: vesa
(II) Primary Device is: PCI 00:0e:0
(EE) No devices detected.

Fatal server error:
no screens found
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Hardware is a Umax S900 dual-200MHz PowerPC 604e (PowerMac clone) with the stock ixMicro/IMS TwinTurbo 128+ (imstt) PCI video card.

Attachment of full Xorg.0.log, xorg.conf, outputs of uname -a, lspci -vvnn, and ddcprobe to follow (xresprobe returns all-null values, just as it had under earlier releases where X worked fine).