  The COBRA collaboration operates a demonstrator setup at the underground
facility LNGS (Laboratori Nazionali del Gran Sasso, located in Italy) to prove
the technological capabilities of this concept for the search for neutrinoless
double beta-decay. The setup consists of 64 $(1\times\!1\times\!1)$ cm$^{3}$
CZT detectors in CPG configuration. One purpose of this demonstrator is to test
if reliable long-term operation of CZT-CPG detectors in such a setup is
possible. The demonstrator has been operated under ultra low-background
conditions since more than three years and collected data corresponding to an
exposure of 218 kg$\cdot$days. The presented study focuses on the long-term
stability of CZT detectors by analyzing the intrinsic, fourfold forbidden
non-unique $^{113}$Cd single beta-decay. It can be shown that CZT detectors can
be operated stably for long periods of time and that the $^{113}$Cd single
beta-decay can be used as an internal monitor of the detector performance
during the runtime of the experiment.
