Daniel T:
Try thinking of it this way. Please let me know if this helps or works... I don't have such facility with the jhanas as you seem to have so I haven't yet played around with it as much as I would have liked.
Think of it in terms of remembering. To stay in the 8th jhana, all you have to do is remember the 8th jhana. Not a thought/concept-based or emotionally-based memory, though, but rather, mindfulness - just that bare awareness of the mind. So another way to say it would be that you have to remain mindful of the 8th jhana - but keep in mind the connection with remembering. (A way I found to activate that quality of mind is to ask myself, "Will I remember this moment later?")
You noticed that 8th jhana tends to flux in and out in 5-45 second stretches. Think of this as remembering the jhana (when you're in it), and then forgetting it (taking you out of it), then remembering it again (back in it), etc... until you forget it more grossly (downshift to 7th after 10 minutes or so).
As you noticed, thoughts take you out of 8th jhana... I think that is cause self-reflective thoughts of that nature are precisely what the 8th jhana transcends (aka formations). Formations re-arising take you out of it ("oo look I'm in the 8th jhana" - by then you're already out), formations stopping take you back in. Because of the unreleased mind's penchant for craving and clinging in general, and to formations in particular, mindfulness in this spot can feel like forgetting. You are basically forgetting the 'self' (forgetting formations), in that you allow them to cease for a period... until they re-arise 5-45 seconds later. Then you have to re-muster up the intent and remember the 8th-jhana-inclination again and off 'you' go... for another 5-45 seconds.
To get down to the practice advice: simply try to go for longer and longer 'gaps', aka periods where you are mindful of the jhana and have (mostly) forgotten the self. It's kind of like allowing yourself to phase out. You just have to let yourself do it. Let go completely, to the point where you don't even care whether you come back anymore. Dare yourself to let go for even longer than you did last time.
This is a great exercise cause it points out that mindfulness is not based on thoughts/formations - it precedes even those. Learn how to access that quality of mindfulness even when walking around and not anywhere near the 8th jhana, and that should prove fruitful.