Update on my issues.
To recap, IN-80 ran fine for 4 months then noisy as hell and randomly filling skimmer cup with water.
Last night I pulled everything apart. Like every tube, everything. I cleaned the heck out of it all.
Problem 1 Motor Noise: After pulling the motor and soaking it in vinegar and then cleaning it meticulously, I found the bushings to be in good shape and after setting back up, it runs MUCH quieter. 12 hours later and it's quieter yet. It's still the noisiest motor I have, but it's tolerable for now.
Problem 2 Cup overflowing randomly with water: This turned out to be a huge piece of salt creep stuck in the venturi "cap" thing that attaches to the motor. I cleaned the hose by running water through it, and cleaned the "L" fitting on the standpipe using a small hose brush (pipe-cleaner thing). I tried to clean the venturi cap using that same pipe cleaner thing, but the angle of it was too sharp. What finally did it was sticking the venturi cap in hot water, and then taking the other end of the airhose in my mouth and rapidly/forcefully forcing water in/out of that cap. It broke loose a HUGE chunk. I was amazed. (just don't suck the water all the way into your mouth & rinse it all afterwards).
When I set everything back up, I left off the return pipe and just run the elbow into the water for returning water to the sump from the skimmer. This alleviated a LOT of back pressure, so my bubble level will need tweaking over the next week or so. I started by closing off the air tube about 40%. This morning I am noticing big bubbles are finally breaking over the top of the cup. This will need to be tweaked to raise the bubble level just a tick more, but again that's a set/wait/set/wait thing.
Anyhow, sorry for the novel. Hope this helps someone!
-Jeff