Mine never gets thick, as it's 85% PG, but it does get darker, thru the day, and sometimes I do experience a slight burnt taste. But it also gets darker in the bottle. I think a) time, and b) oxidation, are probably the culprits. But what's in the tank, I think it does get slightly burned sometimes, especially with a bottom coil and you let it get too low before refilling.
Andria