Video mode instability on Inspiron 6400 with and WSXGA+ display

[Boris Gjenero]

My Dell Inspiron 6400 has a ATI x1400 video card and 1680x1050 WSXGA+ 
display. I have observed this problem in various versions of Ubuntu and 
in Android x86. Most recently I observed it in Ubuntu 17.04 Zesty Zapus 
with 4.10 kernel.

Sometimes after a mode change, the display becomes unstable. This may 
involve:
- white lines flickering to the right of some sharp contrast edges in images
- vertical bars coming from the bottom
- large areas of the screen flickeringly appearing in the wrong location
I don't think it's ever one kind of corruption only. If one happens, 
other kinds will also happen shortly.

When booting up, if the problem occurs, first signs are visible after 
the kernel switches from true text mode to native resolution graphical 
text mode, before X starts running. I know this is a mailing list for 
the X driver, but I'm under the impression that kernel driver and Xorg 
driver development work is done together. Sorry if I'm posting to the 
wrong place.

The problem absolutely never happens in Windows 7. It also absolutely 
never starts or stops on its own. My usual workaround is to use xrandr 
to switch to a different resolution and then back to the native 
resolution. Turning off the display via "xset dpms force standby" and 
then turning it back on or going to sleep or hibernation and then 
resuming can also make corruption start or stop. The probability of 
corruption is low enough that I can put up with it and use the 
workaround, but it would be nice to have a real fix.

There is absolutely no sign of any corruption occurring in video memory 
or rendering errors, and it seems to purely be an issue with 
communication between the graphics card and display. I've read something 
about PLL issues and I'm wondering if this is PLL instability.

Thanks for reading,
Boris