if any module loses signal it returns to the default, for the rpm needle this will be 0 revs.  So if there was a fault where it lost signal it wouldnt hover.   
  
The needles within the cluster are smoothed by the mechanism being filled with a silicone based lubricant gel.  It's a delicate mechanism that shouldn't be left upside down for prolonged periods, the gel can be disturbed leading to sticking needles.  Is your car an import? if so i imagine the cluster will have been removed to alter it to mph?  in which case they may have had the cluster face down potentially damaging it? 
  
I suppose the odd one will eventually develop a random fault, maybe moisture, maybe something else.  It's difficult to guess tbh but thats the best i can think of.