Internal screen vertical interlaced

Just out of nowhere my internal screen now and than turns out vertical interlaced. Either with black or white lines, alternating between normal screen at one vertical pixel row and a black/white line on the other. Like the interlace filter from the old days.
It's seems quite random, no heating issues or special software and the MBP is mostly sitting on my desk.

The mac keeps on functioning and the external display doesn't show the problem.
PR-ram/SMC reset didn't resolve the problem, it just stops and shows up again after a while.
Any idea's what could be wrong? I saw another post covering the same problem, but it wasn't solved.

Thanks!

Sebastian.