Great another Guitar Builder... Sweet! I build too. Checked out your site... Kind of obvious that yer an Ibanez fan.
I just got rid of my last RG570 this week, no more Ibanez in my stash. I'm almost down to using only guitars that I've built myself, still got a couple of factory guitars that I'm keeping though.
A couple of questions for you...
1. Do you know the torque rating of the motors that you have now?
2. What is the Motor Power Supply Voltage?
3. What stepper motor drivers is the machine currently running?
4. What kind of leadscrews are on each axis? (Acme, ball screw, or other)
5. Have you tried a different Control software? Don't know anything about CNCZeus so I cant comment on it's reliability as not many people use it.
I suspect that you are loosing steps for some reason. I could not watch the video but it sounds like your acceleration ramp is set really steep, that could cause some trouble. I doubt that you have mechanical issues unless it's a loose nut mount or loose coupling. (doubt it though)
And that hissing that your motors are doing at idle is just the motors singing from a current chopper doing it's thing.