Anytime you get gurgling or flooding, it's going to be an issue with the head. I've found the build quality of both the stock eVod heads and the stock protank heads inconsistent at best. Sometimes you get a good one, sometimes not. Most of the problems seem to stem from the loose "flavor wicks" that sit on top of the coil. Those assist in feeding juice to the coil while the center pin of the head acts as a reservoir to keep any excess juice out of the airway and let it "drip" back down onto the wicks between draws. I've seen heads with two pieces of wound silica, heads with three pieces of wound, heads with one wound and one shredded... you just never know exactly what you'll see when you pull that center pin out.
Which is why pulling that center pin out and tossing those suckers from the get go is an option worth considering. For new heads, I replace them with 3-6 strands (usually 4 does the trick for 50pg/50vg, adjust up or down from there) of bamboo initially. If I get gurgling at any point, I stick another strand or two in the next time I clean or refill. If I don't feel like it's wicking fast enough, I remove a strand or two until it does what I want it to do. At this point, I know exactly how many strands each unit gets for whichever juice I'm going to put in it. My stainless eVods have more wick, my black ones less. I'm kinda OCD about it.
Also, with the protank, if you restrict too much airflow at the 510 stem, you're probably going to gurgle at some point. Think if you tried to drink a milkshake through a straw with a hole in it. The larger the hole, the less of a chance you're going to get any milkshake. Plug that hole up or make it too small, though, and you're gonna end up with a mouth full of milkshake. The gurgling is a result of excess juice buildup in the center pin of the head due to either not enough wick to soak it up when it drips back down, too little airflow, or a combination of the two.
As for Ocelot's dilemma, I would guess that a cold protank acts as a tiny refrigerator for the juice within, increasing viscosity to a point that wicking becomes difficult and more juice is drawn into the center pin until it eventually floods. That one is not so easily solved. I thought you were gonna knit little sweaters for them?
