Try lowering the CV Dist tolerence In Mach go to General config and in the CV Control section and "CV Dist tolorence" to lower number and see how it goes.?. . . .OR . . . Try setting the "stop CV on Angles>" to 90 deg.
Both will have an affect just in different ways depending on the G-code.

If it works then basicly your trying to go too fast round the corners.? Go read this . . http://www.machsupport.com/docs/Mach3_CVSettings_v2.pdf