Applying in situ process diagnostics, we identified several process drifts occurring in the parallel plate plasma deposition of microcrystalline silicon (µc-Si:H). These process drifts are powder formation (visible from diminishing dc-bias and changing spatial emission profile on a time scale of 100 s), transient SiH4 depletion (visible from a decreasing SiH emission intensity on a time scale of 102 s), plasma heating (visible from an increasing substrate temperature on a time scale of 103 s) and a still puzzling long-term drift (visible from a decreasing SiH emission intensity on a time scale of 104 s). The effect of these drifts on the crystalline volume fraction in the deposited films is investigated by selected area electron diffraction and depth-profiled Raman spectroscopy. An example shows how the transient depletion and long-term drift can be prevented by suitable process control. Solar cells deposited using this process control show enhanced performance. Options for process control of plasma heating and powder formation are discussed.