<div>Maybe something like this for position:</div>
fade = effect("Fade Time")("Slider");
hold = effect("Hold Time")("Slider");
upperLeft = [50,50];
lowerRight = [thisComp.width-50,thisComp.height-50];
period = fade + hold;
seed = Math.floor((time - inPoint)/period);
seedRandom(seed,true);
random(upperLeft,lowerRight);
and something like this for opacity:
fade = effect("Fade Time")("Slider");
hold = effect("Hold Time")("Slider");
period = fade + hold;
t = (time - inPoint)%period;
linear(t,0,fade,0,100
Adjust upperLeft and lowerRight in the position expression to define the rectangular random area.