The chemistry inside the burner must have changed - never seen such a constant behaviour with an HPS lamp.

Probably depleted Sodium

I've seen high xenon pressure HPS lamps do this at the end of their life.
But regular HPS lamps usually have excess amount of sodium-mercury to last without losing all the sodium, like in the case of non-cycling HPS lamps, which have very small amount of sodium-mercury amalgam, so that they ends their life bluish white. So that the mercury pressure increases as the sodium being lost from the arctube, causing lamp voltage rise, eventually cycling.
This lantern right now don't working anymore.