The authors report on an experiment in a double-plasma machine where a positively-biased plane Langmuir probe situated in the target chamber, can excite a high-amplitude, coherent instability in the ion-frequency range when an ion beam from the source chamber is directed to it without reaching it. The ratio of ion-beam to plasma density is around 0.3. In this case the electron branch of the probe characteristic 'breaks off' after the second knee due to the ion beam, and saturates, By increasing the probe bias further an ion instability appears, which is associated with a further drop in the averaged probe current, Under certain conditions the instability shows an intermittent behaviour, i.e. fluctuates without any obvious external influence.