I, too, have a Harbor Freight cabinet. Their cheapest stand-alone model. I haven't used glass beads in it yet, only sandbox sand from Home Depot. Works like an absolute champ, but only if you keep it very full. The more submerged the pick-up tube is, the better it works. In fact, for a while, I didn't have enough in it, and it was erratic like you mention. I finally just dumped more sand in it, and never had another problem.
I use a shop vac with it to help keep the dust down. I aim the discharge of the shop vac well out of the garage and don't really have any dust control problems. I still wear a cheap painters dust mask, just as kind of cheap insurance against breathing the sand dust, which, I guess can cause silicosis, a lung problem.
Uses more air than my compressor can provide. That is, I have to stop and let my compressor catch up. Still, mine does a great job.
I get moisture in mine, too, so I use a moisture separator in the line. Works like a champ, too. No more moisture.
Sorry I don't have any more helpful advise. I guess I'd try filling it up more...
