[ubuntu] Video mode issues on Dell Inspiron 6400 with Mobility Radeon X1400 and WSXGA+ display

I'm trying to use Ubuntu on an old Dell Inspiron 6400 laptop with Mobility Radeon X1400 and 1680x1050 WSXGA+ display. I often see corruption after the keel switches to the display's native resolution. After logging in, I can make it go away using: xrandr --output LVDS --mode 1400x1050 && sleep 1 && xrandr --output LVDS --mode 1680x1050

Considering how the xrandr mode changes make it go away and how it only temporarily appears and doesn't get propagated by graphical operations, I think this is a video mode issue. The keel must be outputting a video mode which isn't quite right for the display. I never see those issues in Windows 7, so it doesn't seem to be a hardware problem.

Right now since I found that workaround I'm giving Ubuntu another try on this laptop using Xenial. It would be nice to have a proper fix.