Those annoying blue lines are still there

, but ... a really massive performance improvement in terms of frame rates - very nearly double! Using the BD track as a benchmark, the frame rates were (avg, max, min):
Ver. 15950: 27, 62, 6
Ver. 17582: 46, 103, 5
Graphic settings were identical for each version (time to go and turn them up

).
Thanks guys, a great effort! Now, if you can just have someone look at those blue lines on the horizon ...