Mark,
You may have to reshoot with another camera.
When I first got the nx5u I tried a slomo and found the same thing.
Then I read the owner's manual VERY carefully.
nx5u is a fake slomo. In other words, it achieves the slomotion affect my reducing the number of pixels that it captures per frame. That way it maintains the same bits per second.
John Lenihan
John Lenihan
LeniCam Video Productions
http://www.lenicamvideoproductions.com