What do you mean by temperature? Let's say you have your gas hooked up to a very tiny thermometer. It has a pointer that jiggles and bumps around. What you'd find is that the pointer fluctuations are mostly related to the mechanical evolution of the thermometer itself, not the system under consideration.
But that's beside the point. Temperature is not just a number on a machine, but rather a concept that we aspire to define and approximate better and better. So really, what is the essence of the concept called temperature?
What we want most from temperature is the notion of equilibrium. Most importantly, two systems at the same temperature should be unaffected when brought into thermal contact. Now, from microscopic mechanics we realize that systems will certainly exchange energy when brought into contact, and so it would seem that it's impossible for the systems to not affect each other. From this it would seem that temperature is fundamentally a flawed concept and cannot apply to microscopic systems.
Statistical mechanics solves this conundrum: what if nobody precisely knew the states of the systems beforehand, that they were randomized? What if system A and system B have states that are so uncertain that when they are brought into contact, their probability distributions are unchanged? For all purposes, there would be no effect by bringing them into contact. Nobody would notice the difference.
In this way two systems may come into contact and certainly affect each other, and certainly exchange energy, yet still be said to be in equilibrium. This allows the concept of temperature to apply to any system, provided the uncertainties have a certain form. However we must then abandon the concept that temperature is a property of a system. Temperature is unlike energy or momentum, in that if you fully describe the state of a system then it ceases to have a well defined temperature. It is only once you begin to speak of average behaviours, of uncertain systems, that the notion of temperature becomes valuable.