OK - my issue was that whenever my router plunge too quickly, the brushes on my dust shoe would push engage the surface of my material with too much force, and the z-axis would immediately jump up about half an inch, while still continuing the job.
I followed the instructions here - http://www.shapeoko.com/wiki/index.php/Operation_Troubleshooting#Belt_.28SO3.29 - and removed one of the springs (my belt tension was already pretty good). Since then, it’s been working great, but having just the one spring makes me a bit nervous, and I’d rather have two weaker springs than one stronger one. (Please let me know if that’s foolish).
Do you have any recommendations for comparable but weaker springs? A link to McMaster Carr would great, but even the specs for the springs that are included as part of the z-axis build would be awesome.