It is experimentally demonstrated that the inherent instability in short pulse, fibre loop, soliton lasers is associated with a resonant exchange of energy which takes place when the overall fibre loop size is comparable to that of the associated soliton period. The instability is characterised by the apperance of spectral sidebands, which can extend to several orders. In the time domain the instability can lead to multipulse outputs and chaotic or quasiperiodic behaviour.