rough circle

Hi all,
got a 3 axis servo driven gantry, rutex + deskcnc, it's working
ok,but when doing full or half circles using the cam generated toolpath
the finish is not good at all, if I slow down the feedrate to about
1000mm/m it gets better but still not good enough.
I'm wondering if deskcnc does not have the necessary speed when using
two axis the same time or it's the servodrive is not fast enough to to
correct the possition.
It's not facets, it's more like the tool gouges in as if I was using
a dull endmill.
It has a 2.5 lead leadscrew 500 ticks encoder, and x4 multiplier on
servo drive.
so, it's a 0.005mm per step with a max speed set at 3800mm/min.
that's = 200 steps per mm.

Any sugestions.