Complex sentence: The lack of equalising pulses in the vertical blanking period of the 405-line system often resulted in imperfect interlacing, which manifested itself as increased flicker and poor resolution when the two fields per frame failed to line up correctly.
Split into simpler sentences: The lack of equalising pulses in the vertical blanking period of the 405-line system often resulted in less than perfect interlacing. This less than perfect interlacing manifested itself as increased flicker (and poorer resolution) when the two fields per frame failed to line up correctly.