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fc2, xorg, 2.6.x, scheduling latency peaks

Hi all. I'm trying to track the cause of high scheduling latency peaks
in FC2 that make the system unusable for low latency audio work. 

Test systems: PIV laptop, radeon video chipset, AMD64 desktop, radeon
video chipset. How I test: I run the Jack (Jack Audio Connection Kit)
sound server with 2 or 3 128 frame buffers for low latency operation
through the Qjackctl gui front-end. I then start GUI apps that use Jack
(for example Freqtweak, Hydrogen and Jamin). I see plenty of buffer
xruns of varying durations during the app load process and afterwards.
The same (laptop) system booting FC1 + XFree + 2.4.26 + low latency
patches has rock solid performance and no xruns in the same conditions.
If I boot FC2 into 2.4.26 + low latency patches I _still_ see xruns, so
it looks like the kernel itself is not triggering them. At least not all
of them. 

I have no hard data but the xruns appear to coincide with X activity. If
I turn off video acceleration I see _more_ xruns (significantly more).
If I switch to the vesa driver I still see xruns (so it would seem that
the radeon driver itself is not to blame). 

Does anyone out there know if something has changed between XFree and
xorg that may account for this change in behavior?

Thanks for _any_ help.... (I also tried several other configuration
options for xorg.conf with no effect whatsoever[*]). 

-- Fernando

[*] and even tried to downgrade to FC1's XFree86 just to see if I could
isolate the cause but the result was too unstable and messy to draw any
conclusions :-)

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