online detection of plateaus in time series
Why don't you use a Shewhart Control Chart (more modernly referred to as 6-sigma) with a moving variable length window to understand the process that you have. 

As you find your process variability decreases markedly, by having too many results within $±1σ$ then you have a "process change" and that might be defined as a plateau. You will need to decide how many observations you require to establish a plateau.

The as you find your variability increases and your $±3σ$, you know you again have a process change and need to recalculate your limits.