I have been observing periodic but severe fluctuations in MCS value despite a very stable RSSI. What could be the possible reasons for such fluctuation. Can someone also enlighten me the logic of how this adaptive MCS is implemented
code states change based on error rates, if a particular code state is having to retransmit packets to frequently it will drop code states and from time to time with attempt to step up to a higher code state. if your are seeing a large amount of retrasmitted and or packet errors, you've got some sort of intermitten noise, like an off channel interferer overlapping you. some other possible issues maybe more reuse CIR, re-use settings not quite right.
if your able to get a clear spectrum analyser grab, it will expose some noise if any is bothering your link, if its AP side, and you don't want to use the analyser, the ACS does a pretty good job finding noise and giving you a graph to look at and aviod it.
if you can tell us a little more about your situation, we can give you some more advice.