Boiling water collectors with an internal two-phase thermosiphon and buoyancy driven circulation are apt for process steam generation in the temperature range 70-150 degrees C. Exploiting the advantages of boiling heat transfer requires mastering the instabilities of two-phase flow natural convection. With electrically heated mock-up test devices the boundaries for stable how conditions were investigated. Between these boundaries, periodic mass flow fluctuations occur and may stimulate resonances of collector components. The fluctuation frequencies measured with the collector mock-up were below 1 Hz. The origin of the oscillations is still under investigation.