I think there might be a hint to the cause of the problem.
I own the SP4 for almost a year and a half now, no issues at all.
I am now travelling in EU, and a little while after I plugged it in in here, the flickering issue started.
If I turn off the Surface and turn on again, I get a few minutes of properly working time, then the flickering starts periodically, then constant.
I'm gonna contact Microsoft, hope they can resolve the issue.