From what I know and from what I have seen while playing with this opsl is that the SHG crystal seems to drift and starts to poison the optical resonance. I notice that as the temperature shifts, the power fades off at on extreme or the other but in this range, the power goes up and down as it degrades. So, 50mw at the sweet spot and then 5mw, then 30, then 1mw, then 10 until there is nothing. So, to me, it shifts enough to disrupt the resonance until it just cannot lase.
Someone who knows more can certainly correct me if I'm wrong but that's what it looks like to me.