  We study a phenomenological model for the continuous double auction,
equivalent to two independent $M/M/1$ queues. The continuous double auction
defines a continuous-time random walk for trade prices. The conditions for
ergodicity of the auction are derived and, as a consequence, three possible
regimes in the behavior of prices and logarithmic returns are observed. In the
ergodic regime, prices are unstable and one can observe an intermittent
behavior in the logarithmic returns. On the contrary, non-ergodicity triggers
stability of prices, even if two different regimes can be seen.
