The intermittent velocity fluctuations, induced by a change in the vertical location of the impeller, associated with the double- to single-loop transition in stirred vessels are thoroughly analyzed. It is shown that this type of flow-pattern instability is characterized by a multiscale and nested structure of the velocity fluctuations, in which intermittent bursts pertaining to the two regimes are intermingled in a seemingly "self-similar" way. A filtering procedure based on a Markovian approach grounded on the definition of a transfer probability matrix is proposed in order to deconvolute from the original time series the nested intermittent bursts corresponding to local single- and double-loop patterns. This technique permits to define and estimate macroscopic statistical properties, such as the average burst duration in one of the two-regimes, and the frequency that characterizes this type of flow pattern intermittency. A heuristic analysis of the dynamic origin of the flow-pattern transitions in terms of global bifurcations and attractor crisis is addressed. (c) 2005 Elsevier Ltd. All rights reserved.