SOLVED!!! By a suggestion from the guy on the Intel forum.
The trick is to disable "Panel Self Refresh" in the Power settings of the Intel Graphics Control Panel. I had tried that, but it turns out it only takes effect after rebooting (which it doesn't say).
The hours I spent on this... Whoever thought that enabling 'panel self refresh' was a good idea? I guess this can be added to the list "thing you should immediately disable after buying an XPS", along with the incredible Adaptive Brightness Control (ABC) in the BIOS.
Alan, thanks for your time. Both issues are now solved: the lines in the color bars are just a weird software glitch in SupportAssist, and the artifacts disappear when disabling 'Panel Self Refresh'. However, I think a lot of time and frustration for other users can be saved if this is somehow let known to them, or if 'Panel Self Refresh' is disabled by default for the next driver update. (I am still curious what the possible advantages of panel self refresh are.)