Monte, Another two cents worth. I am running basically the same setup you have (different controller though). Watched your utube demo and the miss sounds more mechanical to me (but it could just be my ears - selective hearing and all that). My Taig runs very true, excellent repeatability, etc. I was having a problem before that turned out to be misadjusted gibs (too tight). I would lose steps while cutting circles and get ovals. The bind would generally rear its head when cutting arcs. Once I adjusted the gibs it wasn't a problem any longer. Just something else to check. One more item - how about the couplings between the motors and your screws? Some slop there would definitely cause problems.
If you Like, I can send you copies of my configs, motor tunings, etc.
Good luck. Taigs are great, but can be a little (lot!!) frustrating at times. But, the good pieces I have made have compensated for all the headaches. If you haven't already done so, look up Nick Carter's Taig site. There is a lot of information regarding tuning Taigs that I have found really helpful.
Bob@BobsShop- Time to go cut some more.