The concept of acoustically stable and unstable intervals to structure continuous speech is introduced. We present a method to compute stable intervals efficiently and reliably as a bottom-up approach at an early processing stage. We argue that such intervals stand in close relation to the rhythm of speech as they contribute to the overall temporal organization of the speech production process and the acoustic signal (stable intervals = intervals of reduced movement of certain articulators; unstable intervals = intervals of enhanced movement of certain articulators). To test the relationship of stability intervals with speech rhythm we investigated the between-speaker variability of stable and unstable intervals in the TEVOID corpus. Results revealed that significant between-speaker variability exists. We hypothesize from our findings that the basic segmentation of speech into stable and unstable intervals is a process that might play a role in human perception and processing of speech.