I have had fairly significant issues with this since I got my Mk25/S600.
Today I did a LONG shallow dive (over an hour) and got the usual cotton mouth. So I switch to my backup reg (G250) for grins and giggles. Surprise! The problem was significantly lessened.
Now what's the difference? The G250 has a metal air tube. The S600 is ALL PLASTIC.
I'm going to swap my second stages for two dives tomorrow, and see if it makes as big a difference as I saw today.
I bet it does.
If so, then my recommendation would be to get rid of all-plastic seconds for those who have issues.
Frankly, Doc, I bet that if you start looking at what the airtube is made out of between those who have and who don't have significant "cotton mouth" issues, you'd find some correlation.
The difference, for me, caused by simply switching to my backup was STRIKING.