The effect of thermal cycling upon the thermoelectric performance of state-of-the-art p-type half-Heusler materials was investigated and correlated with the impact on the structural properties. We simulated a heat treatment of the material similar to actual applications in the mid-temperature range, such as occurs during the energy conversion from an automotive exhaust pipe. We compared three different compositions based on the (Ti/Zr/Hf)CoSb sub(1-x)Sn sub(x) system. The best and most reliable performance was achieved using Ti sub(0.5)Hf sub(0.5)CoSb sub(0.85)Sn sub(0.15), which reached a maximum figure of merit ZT of 1.1 at 700 degree C. The intrinsic phase separation and resulting microstructuring, which are responsible for the outstanding thermoelectric performance, were stable even after 500heating and cooling cycles. The standard deviation of the obtained ZT values lies within 2-3%, which is significantly smaller than the measurement errors. Towards application: We have simulated the heat treatment similar to actual applications in the mid-temperature range such as in an automotive exhaust pipe. Even after 500heating and cooling cycles the intrinsic microstructure is stable. The material Ti sub(0.5)Hf sub(0.5)CoSb sub(0.85)Sn sub(0.15) shows the best and most reliable thermoelectric performance reaching a maximum figure of merit ZT of 1.1 at 700 degree C.