I am having an unusual issue with this CMOS CD40106B Schmitt inverter. I am having several pieces fail in the same failure mode. Out of 6 inverters in this package the same gate fails in the same manner. If I have a 15VDC high or 0V low, the output is as it should be, the opposite state. If I put a pulsed waveform on the input, specifically +15V going to 0V for 1 millisecond with a repetition of 100Hz, my output at pin 10 is always low. These failures do not happen immediately, but are customer product returns. What could possibly be causing this failure mode?