I need something like dan ebberts expression :
holdTime = .5; //time to hold each position (seconds)
minVal = [0.1*thisComp.width, 0.1*thisComp.height];
maxVal = [0.9*thisComp.width, 0.9*thisComp.height];
seed = Math.floor(time/holdTime);
seedRandom(seed,true);
random(minVal,maxVal)
but instead of evry .5 sec, evrytime light intensity is >= 20