I also upgraded to version 2.3 of Mach3 from the previous lockdown version and am having the same problem # 1. When the direction of cutting in the x-axis or y-axis reverses, the feed rate briefly decelerates then accelerates back up to normal. Any subsequent move in the same direction is fine and moves normally, but when the direction is reversed the problem re-appears for the first move again. This happens for me while jogging the x and y axis'. The z axis doesn't seem to be affected.
I have noticed there is a newer version 2.42 available so I will see if it fixes the problem.
My axis' ran fine with a version prior to this.