Need to stop abrupt accelerations, on direction changes. Puts everything out of alignment. 4x8 Blackfoot

The acceleration setting in Mach3 or other control software you may be using should be relatively low. For the blackFoot, we don't exceed a value of 20 for acceleration. To modify the acceleration in Mach3, go to the motor tuning option under the config menu. Make sure to use exact stop rather than constant velovity if the velocity is set high so the corners will not curve. Having a high velocity and low acceleration, constant velocity will cause the corners to become rounded. With exact stop, the outside corners will be as intended and inside corners will be the diameter of the end mill. Additional Information: I was unable to add that I am using PlanetCNC. My understanding is the USB breakout board is only planetcnc compatible. I have changed accelerations to a minimum (was only ever 10, now at 1) and it still "jumps" fairly randomly. I have lowered speeds to absurdly low. To slow without burning out bits if I was actually cutting. Still Y axis particularly jumps from speed of 300 to 2000 or more. I am using mm. I set the bit feedrate to 30mm/sec. The sudden jumps only ever seem to happen at the start of a cutting program. 5-10min in and it settles. With little or now sudden accelerations. I am not using constant velocity. Any guidance appreciated.