fs = 30     # frame step value
fr = 10     # random frames added to step position

the frame step value sets up every 30 frames (1 second)
the fr value adds from 1 to 9 frames to that number so a light may change at 34,68,92 frames for example

if you set fr lower than 10 but at least 2 the lights will change closer to each other
if you set fs lower but at least 1 , there will be more changes during the anim.
setting fs = 1 and fr =2 will cause a change almost every frame

I would not try lower that fs = 5 and fr = 2  and then increase

The way this uses randoms there will be times the light does not change. Its like flipping a coin, sometimes you will get three heads in a row .