After further thought, I might have an inkling of an answer to Xtol's sudden failure.(posted 8648 days ago)
I've heard Ascorbic acid referred to as a 'reducing sugar'. Now, if it really is a member of the sugar group, then it's ideal food for bacteria and natural yeasts.
Perhaps Xtol dies because of microbial attack? Seems like a reasonable explanation for sudden death.
Anyone fancy adding some penicillin to a batch to see if it extends the working life?