Actually, I don’t think you even need the markers. If you set the precomp up so it is the exact length you need to encompass however many 5-second clips you have, then time remap it in the main comp and drag the outpoint to the end of the main comp, this time remapping expression should take care of it:
dur = 5; // play each clip for 5 seconds
seed = 1103; // change this to get different results
nTot = Math.floor(source.duration/dur);
n = Math.floor(time/dur)
seedRandom(seed+n,true);
tStart = Math.floor(random(nTot))*dur;
tStart + time%dur