Hi Wozza2014
I should have mentioned in my reply here that my NUC is a D54250WYKH which is a 4th Gen i5, and i have this problem.
So when you say you think your CPU is not powerful enough for 1080p flash, i think that's not true. Since NUC's with i5 have the same issue.
I have used PC's with lower CPU's which can perform this task, so i think the problem is with the NUC.
This is just my opinion.