Picture hangs on Ubuntu 15.10 MATE with 2 video cards and 3 monitors. What could be?

Question

NVIDIA

Yes:
OS: Ubuntu 15.10 MATE
Proc: Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz
Mother: Gigabyte GA-Z68AP-D3
PSU: Chieftec 550W
HDD: 3 x 1Tb HDD WD + 60 Gb SSD
Video: 560GTX + 9800GT (no SLI jumper)
Driver version: 340.93 (352 does not support 9800)
Xorg.conf config: pastebin.com/8G9dkXiH
Question:
After login (in lightdm) the MATE desktop is shown and the image freezes. Only the mouse moves. The machine remains alive - you can go to the console by CTRL + ALT + F1-6. If you remove the second viduhu from the config (line 4 in the config) - it works on 2 monitors without problems. Nothing in the Xorg and LightDM logs. A similar configuration on Linux Mint 17.2 MATE works with 4 monitors without issue. I sin on drivers with too much "age difference" of video cards and on the lack of power supply (after the second installation of 9800, the machine did not always start up the first time, I believe that there is not enough database to run all the screws with the video cards). Where to look?
PS: Yes, I know about LTS. No, I'm not going to change DE :)

Answer 1

you can try to change the parameters for loading the kernel:
replace /etc/default/grub line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
with
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.semaphores=1 i915.i915_enable_rc6=1 pcie_aspm=force i915.lvds_downclock=1 i915.i915_enable_fbc =1"
well, although everything is right here all at once) you can start with i915.i915_enable_rc6=1" and i915.semaphores=1
and after that sudo update-grub
it still probably makes sense to change the X mode in /etc/X11/xorg.conf
in the Device section add
Option "AccelMethod" "uxa"
most likely there will be a sna method