Hi there,

your cutting prob sounds like a mechanical rather than a motor power issue. I would suggest that under cutting load the integrity of your frame is not being maintained in the Y axis... causing it to distort and making the y axis bind/stall and lose steps. Straight cuts impose some force on the axes but if there is deflection it's primarily in line with the axis (shearing in a plane) so you don't see so much binding. also lost steps are potentially disguised as the next step will make the cut anyway... but a non-straight cut imposes the force in many different directions and you get twist effects which can make the axis bind when a straight cut doesn't, then when the cut changes direction the twisting is reduced, the axis unbinds and moves again but with the lost steps it is out of position.

you can test this out by running a very shallow circular cut on some old MDF which is nice and easy to cut as well..... start at a really low cut amount in the 0.5 to 1mm range.. run your spindle at high speed and use a slow feedrate. See if you can cut the circle complete and check that it is a circle. If so double up the cut depth and run again... repeat until the problem manifests, you may see the beginings of it with the shape of the circle changing. This will show if the problem is down to deflection as you increase the load on the cutting tip... it will also determine a "functional" setup for your machine as it stands, you should be able to arrive at a workable comprimise to avoid deflection and lost steps but still cut at a reasonable speed...

hope this helps.