Hi Jbarbaro!
I'm glad you are enjoying the VK!
I just saw this thread and thought I'd chime in... I have had that happen to me a few times as well. The premium cartos are pretty sweet, but they are designed to run at a higher temp than standard cartos on a given voltage. Using them on a PT at higher voltage makes them even HOTTER. In my opinion, prems aren't a good match for the PT because of this. This is relevant in your situation for a few reasons: 1. Although you had been topping off regularly, the juice near the coil can dry fast. The temp at the heating coil can cook it before it all gets vaporized, and it makes those little rocks you are talking about. (This is less of an issue at 3.7V). 2. Another thing that happens when they run so hot is that they can boil the juice at the coil and 'spit' it out the mouthpiece at your tongue, along with those juice rocks.
It sounds like this is exactly what happened, and my experience matches yours closely. This is relatively rare, but as pointed out it happens A LOT more often when using a premium on a PT.
I find that the premium cartos shine their brightest on a battery, and the standard cartos work best on the PT. Since I've been using them this way, I haven't had that happen again and my overall experience has been fantastic. As an aside, I also think the flavor gets destroyed a bit when using high voltage in conjunction with a premium carto (which after all was designed to give you that higher volt 'sensation' when you use them on a battery).
Anyhow, enjoy the cognac and watch out for those tongue scorchers!!!