So, got it built, everything connected using Mach3...slaved A to the Y axis. X on gantry is silky smooth, and one direction on y, towards the side that the motor is on....but the other way, in the direction that the pulley that connects to the other side of the gantry (Root3 calls this the motion link), it is very jerky and struggling....and eventually, it gives up and then the jog in both positive and negative jog the gantry in the direction of the motor side....
I don't think it's mechanical...the belts are pretty tight (is there such a thing as too tight?). Without power, there is equal resistance pushing the gantry manually in either direction. If I restart Mach3, it resets and will try again, until the Y jog directions both go only toward the motor side (call that the smooth direction)....
I will say that the motors and drivers are Aliexpress specials, so not highest quality (wanted to learn before spending too much)...I am using the same RNR board on the buildyourcnc.com website and videos, and it seems to be able to power much larger machines.
Any and all help appreciated.