mach64GX, clock problem?

[Ed Siefker]

I have a mach64-GX that keeps trying to overdrive
my display.  I've set a modeline of 640x480 at 60hz
and 29.8khz.  When I run it, my display complains
that I'm sending it 239Hz and 118K.

This is a factor of 4 difference in both frequencies.

What's likely to be happening here?  I can very
occasionally get it to display, but it's monochrome
and stretched.  I wouldn't be surprised if the stretch
was a factor of 4 actually.

It really sounds like a problem with the clock, and
when I check the log, it's full of modes eliminated
for a bad clock.   For example:

[  5318.178] (--) MACH64(0): Reference clock 157.5/11 (14.318) MHz.
[  5318.179] (II) MACH64(0): Monitor0: Using hsync range of 30.00-85.00 kHz
[  5318.179] (II) MACH64(0): Monitor0: Using vrefresh range of 50.00-160.00 Hz
[  5318.180] (II) MACH64(0): Clock range:   9.00 to  80.00 MHz
[  5318.180] (II) MACH64(0): Not using mode "1024x768 at 75" (bad mode
clock/interlace/doublescan)
[  5318.180] (II) MACH64(0): Not using default mode "320x175" (bad
mode clock/interlace/doublescan)
[  5318.185] (II) MACH64(0): Not using default mode "320x200" (bad
mode clock/interlace/doublescan)
[  5318.185] (II) MACH64(0): Not using default mode "360x200" (bad
mode clock/interlace/doublescan)
...
[  5318.239] (**) MACH64(0): *Mode "640x480 at 60": 23.9 MHz, 29.8 kHz, 60.0 Hz
[  5318.239] (II) MACH64(0): Modeline "640x480 at 60"x60.0   23.86  640
656 720 800  480 481 484 497 -hsync +vsync (29.8 kHz)


I know there's an option to actually set the reference clock, but the
documentation says it's for non-intel platforms, so I'm disinclined to
play with it unless encouraged to do so.

FWIW the entire log can be found here:  http://pastebin.ca/2138566
And my xorg.conf is here:  http://pastebin.ca/2138568

I know this is an old machine, but that's all the more reason why I'd
really like to have 2d acceleration on it.  VESA is unbearably slow.
Any thoughts, advice, hints, tips, or pointers would be appreciated.