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[matej_bb <matej_bb@xxxxxxx>]

I have done a regression test, and the last release that works fine is 6.12.4. From 6.12.191 up, xorg log produces the aforementioned PLL error.
But let me explain further:
I was trying to upgrade "only" because with X700 at full clock, this laptop's fan is spinning almost all the time, even when CPU is idle.
In Windows, I solved it by Powerplay - the "battery" option (lowest clock), since I my apps normally do not require much GPU processing power. That way, the fan kicked in very rarely, almost never!
Now in Ubuntu, I spent many days trying to underclock the X700. The version marking of xf86-video-ati confused me a bit, because there are the 6.12.3 and 6.12.4 releases which do not appear in log view on the project page. There is only 6.12.2 and then 6.12.191. 
According to the logs, I found out that radeon_pm.c file with ForceLowPowerMode was added in 6.12.2, but when I look into the 6.12.2 official release, the radeon_pm.c, nor the ForceLowPowerMode option is there. It is not even in 6.12.3 nor 6.12.4. It first appears in 6.12.191, which, unfortunately, does not run for me because PLL error. And, what is very weird to me, in the subsequent releases the radeon_pm.c and ForceLowPowerMode options are gone! And I was not able to find the reason anywhere in the logs.
Now how I fixed this issue: I browsed the log of diffs, opened several of them before 6.12.191 and looked for the one that actually has the radeon_pm.c. I finally found and successfully tested the version called "xf86-video-ati-efa0825a86a8dc0f03ebb42c576ed26189e9d4bb". Inside this file, it is marked as 6.12.2. (But official 6.12.2 to 6.12.4 do not have radeon_pm.c! ) This version works flawlessly for me, no PLL errors, and most importantly, what makes me most happy is this line in xorg log (attached):

(II) RADEON(0): Force Low Power Mode Enabled

I did make one modification though. I knew from playing with Powerplay in Windows that the card works stable until approximately 105MHz (normally about 350MHz). But in radeon_pm.c, the sclk was only divided by 2, that means the card was working at about 175MHz, still unnecesarily high and noisy for me. So I changed the "sclk / 2" and "mclk / 2" to  "sclk / 3" and "mclk / 3" in the ForceLowPowerMode section. Now the laptop is finally as quiet as it was in Windows! :) 
So besides the PLL error, what is very strange to me is why this ForceLowPowerMode option is not available in further releases after 6.12.191... ?

** Attachment added: "Xorg.0.log"
   http://launchpadlibrarian.net/48340339/Xorg.0.log

-- 
[hd 3650] Couldn't find valid PLL dividers - graphics failure
https://bugs.launchpad.net/bugs/336348
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