Is reduction potential and stability related?
Remember that reduction potentials do not have any meaning by themselves. They are measured against a hydrogen cell to which arbitrarily we assign the value $E^0=0$. You can compare between different potentials. For example, in your case, you could say that that $Cu^{2+}$ has a greater tendency to be reduced than $Na^+$  because its reduction potential is more positive. However, I do not think the potential itself tells you anything with regard to the stability (and, in any case, what do you mean by that) of the species involved in a particular half reaction.