I think I have a similar circuit, a 40106 schmitt inverter wired to make several oscillators, and one of the schmitt oscilators running at a lower freq triggering a data latch.
This was for more than 1 light, I was thinking however you could use 1 o/p to drive your lamp, and another o/p to randomly switch the o/p off altogether to produce your 'off' cycle you wanted.
If you use solid state relays that implement zero cross switching, your lamps will last longer and the system will create less rf noise.