Self-sustained oscillations and fluctuations have been observed in undoped hydrogenated amorphous silicon (a-Si:H) thin films when small portions of the sample were biased using coplanar electrodes. After subjecting the samples to very high voltages several times, the current flow becomes unstable. At constant voltage the current continuously increases ending in the irreversible formation of visible channels between the electrodes which exhibit an almost periodic structure. During this process of pattern formation random fluctuation as well as periodic single-frequency and mode-locked multiple-frequency current oscillations and random telegraph noise have been detected. These phenomena are attributed to microcracks in the sample which change their resistivity by the hopping of hydrogen between different binding configurations.