Finally decided after trying display tweaks, resets, booting other systems from a usb stick, running in safe mode - the problem never goes away - allpointing to a hardware problem methinks so my surface pro 4 is now winging its merry way to MS, Germany under warranty. I've been very impressed by the support routes offered by MS and the ease of the MS online system for submitting devices for warranty repair/replacement. I'll keep you posted.