Sometimes, you will see kind of a "warble" in the waveform when looking at just a few cycles of waveform.
If you zoom out in time, showing, for example, 100 milliseconds per division, or maybe even a bit slower, the repetitive wiggles or ups and downs in the amplitude of the waveform. It's just that it is hard to catch and press stop at just the right time.
You could also set the trigger to just above the 170V peak area to catch the higher voltage cycles. But if that voltage is lower than 120V RMS or ~170V peak, it is quite a bit harder. Also harder to see if the voltage is ramping up and down over several cycles if that is what is happening.
Does this sound possible ?
boB