I upgraded from kubuntu 14.04 LTS to 16.04 LTS and now my screen shows up and disappears

It looks like -flicker-, but contrary to what I could find in Google, it is not to do with Chrome. It is the log-in screen already that shows up occasionally on the screen, and then the screen turns black again. This is the second, main, monitor to my Thinkpad T410s. On the built-in screen everything is fine. But it is so tiny, I do not use it. I also came across some random shutdown troubles during my search. I do not have any such. I would not say that the display flickers, rather the display shows up for spilt-seconds while the larger part of the time the display is simply black.

As last resort: How would I downgrade from here so that I can use the PC again, please?

Here is - on request - the output of lspci -k | grep -EA3 'VGA|3D|Display':

00:02.0 VGA compatible controller: Intel Corporation Core Processor
Integrated Graphics Controller (rev 02)
        Subsystem: Lenovo Core Processor Integrated Graphics Controller
        Kernel driver in use: i915
        Kernel modules: i915