Odd behaviour at high speed

Hi, I am setting up my newly completed DIY CNC (posted earlier in woodworking section), and am noticing odd behaviour of the motors.

I am using the latest version of LinuxCNC and set up the parameters using Stepconfig. I "tuned" the motors as described in the documentation, and arrived at a max velocity of 130 mm/s, with an acceleration of 800 mm/s/s. These settings run nice and smooth in jogging mode.

Since one of the uses uses to my machine is to do vinyl cutting, where there is practically no limitation on knife speed for cutting, I did some tests to see how well the machine can follow drawing circles at various speeds from 450 mm/min to 7200 mm/min. The results are in the pictures (hopefully it is clear enough)

https://www.cnczone.com/forums/attac...d=417616&stc=1 https://www.cnczone.com/forums/attac...d=417618&stc=1

At 450 and 900 mm/min, the path is stable, but at higher speeds, there is a sort of random jittering occuring, where the path erratically goes back and forth, and the location reference is lost (the circle ends don't meet). It is mostly a jitter on the x axis, but the y seems to do it too. The worst jittering occurs at 1800, and less at higher speed. I do not get any Realtime errors being displayed, although I understood problems related to "jitter " are missed steps, and this is not missed steps. Is this actually latency issues? I tried different acceleration values; gets worse at low values, but still present at high values. I am not too worried about this, as 900 mm/s will be fast enough. Still it is odd that the machine will jog smoothly at these speeds, but under programed movement, has this jitter. Is there other things I can test? Or is this a limitation of LinuxCNC? or of the computer?

Some relevant information of my system:
Pentium 4 with 3GHz proc.
1 Gig ram
Latency test gives ~30,000, so entered this value in Stepconfig
TB6600 controllers (those little black boxes you get on ebay for cheap)
NEMA 23 motors
Single stepping
GT2 Belt drive, no gear reduction, drive teeth are 16 and 20 for X and Y, respectively