Nigel,
My recommendation is to buy the 128 gig memory unit. I have 4 nx5us for over a year and have never had a problem with the memory units.
The alternative is for you to try various brands of sd cards until you find one that works reliably. For instance, a sony brand sd card.
The only other question I can ask is did you format the sd cards in the camera itself?
John Lenihan
John Lenihan
LeniCam Video Productions
http://www.lenicamvideoproductions.com