So I finally took the skimmer apart and did a few tests. I used a vacuum pump and applied 5 inches to the venturi hose and plugged the blue screw hole and the air inlet with my finger and it held fine. I figured you meant 5", 5mm is hardly anything, I can make 5" with my mouth. No parts were hurt

Anyways, it held vacuum just fine and didn't budge for a few minutes. So that shoots my theory.
I then put in a shorter pipe with the thin-wall PVC. You can see from the photos its almost 1/2" lower than the water line. The black one that came with the skimmer was right at the water line. You can also see that the water boils over the top of the pipe about 3/8 of an inch. Now there is no gurgling coming from the skimmer.
Do you think the pump is too strong? That's quite a bit of water coming out, more than a 'trickle'. I also get a bunch of micro bubbles, and have since day one. You can see them stuck in the blue sponge. In fact, after cleaning the blue sponge, it fills with air in an hour or two and the water basically flows over it and not through it.
The skimmer has been running about 10 months. You sent me the smaller 60hz impeller and that made better skimmate, but no help on the bubbles. I was thinking of triming down the 50hz impeller I got with the skimmer to see if that helps. I have the blue screw about 2.5-3 turns out from all the way in and am running the foam extractor. It skims very well, but 3 turns sounds like a 9010 cup setting. So I'm thinking less water, more venturi.
Sorry about the blue light, it keeps alage down in my sump
The poor venturi was all salted up from sucking in water: