Hi
just finished a concert edit with half the footage shot on nx5 the other half on my sx and had similar problems , only way i could get footage to work was to reclip in camera down to smaller than 5 minute clips. and still a few would not run correctly ay all which i suspect was an sd card or writing to card fault. personally I think the NX5 is crap, sorry I bought one apart from the above issues even when working its impossible to get really sharp .
anyway try the smaller clip option see how that goes.
cheers