Sounds like the first time was A&P territory, and the second time was also A&P territory. Not everyone has clear A&P Events, but it sounds like the first one probably was, as you know it has happened because the whole thing fades rapidly after it even if one keeps practicing.
Okay, Daniel. [Thank you for your response!] A more delicate question, then: If these were both A&P events, then this would imply two separate "cycles" of insight? - implying magga and phala in between? (It seems absurd to ask this question, but I have never discussed these experiences with a teacher, so I really appreciate going into it a little now with you.) So, what started the 2nd "cycle" of kundalini-type experiences was an experience of what I have always considered my one and only experience of "jhana". I have always wondered about this jhanic experience though, because it only occurred once like that, and even with subsequent seemingly very high levels of concentration, has never since repeated that sort of experience. Which was this: I was practicing Anapana, and the concentration suddenly became perfectly stable, and effortless (after several days of strong concentration, the body stiff and upright, and with that "dimming" of bodily sensations that I was experiencing for the first time, that suddenly made it easy to sit even in lotus posture for long periods without difficulty). It was suddenly perfectly easy to keep an iron grasp on the breath without intervening thoughts - this started during discourse, and continued even walking out of the hall and getting a drink of water, and then while going to my cell. In the cell I sat down and the mind stayed right there. The breath became shallower and shallower and shorter and shorter over a 1/2 hour or so maybe, and I stayed right there with it the whole way, amazed at how automatic and easy it had suddenly become. It continued to become subtler and subtler and clearer and clearer, until I felt I might pass out, or die. I remember a strong sense of surrendering to that death, and shortly after that, something tremendous happened, where the mind "zoomed in" like a 100 times at that spot, and some sort of shift in consciousness occurred, accompanied by tremendous bliss, throughout the body. My surprise gave rise to thoughts, but even as the concentration devolved, that sensation lingered... Is this what first jhana feels like? Or was that something else ie is that what sotapatti fruition feels like? It was not many days after this on the same course that the kundalini-type experiences suddenly began again (after that 4-year gap).
Seeing things arise and pass, happen on their own, and noticing the suffering aspect of them is insight. Many reject their own hard-won insights as they didn't like the sensations or qualities they gained insight into or didn't realize that insight is not always fun or pleasant.
The first time it happened I was very much rolling in the experience; the second time(s) were characterized by much greater insight though: the whole process seemed disgusting, unpleasant, and repulsive, even as it unfolded, and awareness of anicca was predominant.
As to where does the ceaseless rising and passing of dissolving impurities conclude, this should perhaps be rephrased: all sensations arise and vanish complete, and thus they self-liberate by vanishing utterly.
Worth meditating on.
Kundalini stuff can be fascinating and opinions vary as to the degree to which it should be given a special place or just seen as it is. I favor bare insight practice on the sensations that actually make up your reality, all of them, regardless of what they are, but then it depends on what you want to do. Fascination with meditation side effects and raptures in general causes them to linger and thus slow down progress, at least on a pure insight front.
Of course I would prefer to "see it as it is"!

The second time in particular was followed by a huge upheaval of "stuff" and ushered in a very dark period in my life, which coincided with a return to US and starting graduate school. I'd like to think there was some insight in that suffering though, and not just blind flailing (which there was undoubtedly a bit of as well!!)
But it's good to clarify that all this stuff falls into the "raptures and side-effects" category.
Thanks for your thoughts on all this, Daniel.
Sean