having 'ColourFixerPro' on one clip and 'Spotlight' AND 'ColourFixerPro' on the other clip will lead to confused flickering in the cross fade transition period.
both clips are 1080 / 100fps on a 25fps timeline played back in slomo.
removing one effect from the second clip resolves the issue.
at further investigation, it looks like it could be some buffer overrun if forcing preview beyond replay capabilities. 'AUTO' made the problem to disappear.
What was interesting though was that even the rendered output was damaged, and that's what was/is my concern. but yes, maybe it has to do with the video card (ol' lame GT650M, pushin'it)