Lost steps can come from a variety of places. It sounds like you have covered the mechanical parts, although it does concern me that you say it is missing steps at the ends of travel. That could indicate that your lead-screws are worn more in the middle and the nuts were adjusted there. When you get to the end of travel the nuts will be tighter and may bind. If you have a heavy, lopsided piece of stock that could cause some binding as well.
The Xylotex drives are good drives but are limited on performance due to the stepper drive chip they employ. I bought one of their 4-axis systems a few years ago and suspect that 20 IPM is about as good as you will see on a Taig.
Your 166 oz-in motors are likely fine, it depends on their exact specs. I recommend the 166 oz-in I sell for Taigs, larger motors DO NOT improve performance on the Taig. On the Taig in my shop I can do 100 IPM all day long with those motors and my drive box. (http://cnczone.com/forums/showthread.php?t=89536) I generally have my max velocity set to about 40 IPM as I'm not cutting anything that can take advantage of the higher speeds.
Another possibility is that your PC is not outputting a smooth pulse stream to the driver. This can cause missed steps and limited rapids as well.