I do treat the LSN as an idle mixture control only, because that's all it really does.

My best hypothesis at this point is that the carburetor is getting just a teensy bit too hot towards the end of the run, and that's causing the problem. I have no proof though. Or maybe there's an air leak that only opens up enough to cause trouble once the engine is at full operating temperature, I dunno.

Enriching the fuel mixture seems like the obvious solution, but the problem when I do that is it tends to bog-down on acceleration, and has trouble reaching proper operating temperature at all. And I'm not talking about loosening the HSN by a half-turn, or a quarter-turn, or even an eighth-turn; I'm talking about loosening the HSN by the smallest amount I can turn it without the sealing O-rings pulling the needle back to its previous position.