Thanks everyone! I discovered that little hex nut under the head. After tightening with two wrenches instead of my hand holding the belt and a wrench, I can finally cut without random and unexpected Z excursions. Finally!

I also agree that running random speeds/feeds with zero experience with this machine (ie, I'd never know the warning signs that I'm running it wrong) is going to wreck something. 

I suppose what I should do is get a tachometer and set a calculated feed/speed, and use depth to control the load on the machine. I was hoping there was a way to watch the chips and listen for the machine getting bogged down and know to adjust things. Usually when I get into anything that involves math, the formulas get you started. I have no intuition on this stuff yet so I suppose using the formulas is the only way.

I was able to cut at 3 IPM and full RPM, .025" depth cut. From the sounds of things, I should be able to cut even faster/deeper in this 6061 aluminum. I'm using HSS though (not carbide) but I'm hearing IPM upwards of 14 from others.

I've got other accuracy issues with the machine, but at least this issue is more or less figured out.