It should be stable, but money has a "time value." What that means is that the present value of some amount of money has more value today than the future value of the same amount to be received at some future date. That's because in capitalism you can use that money today as capital to realize earning potential. This is even ignoring the risk that you won't even receive the promised money due to unforseen factors.

Regarding stability, intrinsic value/commodity backing adds another factor of instability, e.g. it being profitable to melt coins for the metal.