I do suspect poor cards account for many of the recording issues, when ordering a new card I think it would be prudent to order a class 6 card, just to give that little extra 2MBps headroom in write speed, as although in theory a class 4 should be easily fast enough, you have to bear in mind the write figures are based on continual writing speeds.
My video's record at aprox 17200kbps, which equates to a writing speed requirement of about 2.1MBps, so in theory as class 4 cards should be able to write at 4MBps we have plenty of headroom speed, but IME this can vary depending on the efficiency and design of the device being used to write the files and so may lower this figure.
When these camera's start to re-cycle, it also has to delete it's older files which takes time, and maybe in some instances fails to have the speed to keep up. A decent class 6 gives that little extra 2MBps headroom in speed. If you already have a couple C4 cards then by all means try them, my own C4 cards seem to work okay too, but when buying new cards I think with this camera Class-6 is the better choice, and of course it still runs at the standard bus speed.. maintaining better compatability, unlike the Class 10 which uses faster bus speeds, and which they advise we avoid.
As I stated many times I've had no issue's with Samsung C6 cards at all, though any good brand C6 should also work fine. I use 32gb cards, but its quite common to see higher capacity cards even same brand & class often have slightly lower write speeds than lower capacity cards such as a 16gb.. just something I've noticed.
It's obvious that when it has to continually delete files, this must have a very big impact indeed on the overall writing speed. It's not uncommon to find a good Class 4 perform better or equal to a very poor class 10 card, so 'buyer beware' , sadly it's also true that a good card won't solve everyone's problem, but it definately helps.
EDIT 20/10/13.
I have since experienced a issue with a poor mini-usb connection which was causing one camera to stop recording just seconds after it was powered up, prior to this it just stopped recording on it's own after running all day fine. Cause of this latest hiccup as seen in later posts was nothing more than a poor mini USB connection, a quick blast with isopropyl, and a slight squeeze in of the connector to make it a firmer fit, solved it... so worth bearing in mind.