random chase effect

I'm trying to create a random chase with 5 fixtures. 

I've made a relative effect chasing the selected lights in a beautiful chase from left to right (or from right to left if i hit reverse) but i can't get it to randomise with fixture turns on at first. 

With the random group function it just randomised how many fixtures are turned on. 

I've tried randomising the effects selection (selecting the fixtures and then using offset) that just randomize the first selection.

Its probably very easy, but i just not getting it.

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