This is my understanding, but then I'm no expert. The problem you may have is that most thyristors and triacs use the "zero" switching method, ie in an AC environment they change state as the mains crosses from negative to positive phase (and vice-verce), so you have a 50 / 60 hz pulse to start with, and if you have a small PPm signal on the gate this may cause the flashing / pulsing often seen.