Soulgrower,
Maybe not much consolation, but my P. puchellum is going through a similar ordeal. Yellowing, drying leaves which eventually fall off. Eesh.
However, new leaves seem to be growing in their place, and these don't seem to be affected. Are you getting new growth?
At first I suspected mites (always my first assumption with any downward-spiraling plant) but nothing else seems to be suffering and I can't find any mites or sign.
My plant is also in a greenhouse and exposed to natural light, so maybe there are some hereon unforeseen dormancy issues, brought on by changing, seasonal light cycles? I've also thought it might be due to increased humidity (greenhouse window and doors solidly shut for winter), a change in location and available light (competing with the rest of the crammed greenhouse jungle), or overwatering.
Personally I've cut way back on my watering, and I'm keeping my grubby fingers crossed that my plant recovers fully.
Sorry not to be more help, but I'm hoping it's just a seasonal thing! Please let us know if you find a solution or explanation.