If you are still having this issue I may have a answer for you.
I believe you are running a non-stock kernel? Matr1x?
With whatever method you chose, I use setCPU, raise the voltages by 5% at a time. A reboot is required after raising the voltage's, in fact two or three maybe needed before the change takes affect.
I have had the same issue and after much playing have stabilized my NS4G getting rid of my mounting issues, I can not over clock past 1.2GHz and really only raised voltages by 5% but more maybe needed, if I do it always results in the same un-mounted sdcard/internal storage. It still does un-mount every once in a while but it's so rare now I don't even think about it.
I hope this helps you, have a great weekend!