If you have 1/10-10 screws and 8000 steps per inch, then your Xylotex must be set for 1/4 steps. Try changing it to 1/8 step mode, and set the pulse widths to 2 and 2.
Is it always the same side that stalls? It could be resonance, which there is no simple fix for. Sometimes increasing the accel can help, provided you have enough power to do that.
You can also try decreasing the VREF voltage on the Xylotex to around 3 or so and see if that helps? Do you have a fan blowing on the heatsinks?