Check for mechanical binding by manually moving the axis at different positions. Measure the torque required to turn the lead screw (assuming that's what's driving it). If it changes significantly or is much higher on one axis, look deeper into the mechanics. You can also just run air jobs for hours to wear in the machine.
If it gets worse over time, check the temperature of the stepper motors... if they are getting hot enough to burn, add heatsinks. Seriously... people will poo poo this suggestion, but I've seen it make a huge difference... of course, it only applies when the heat corresponds to the problem. e.g. when cold motors are fine but as they get hot, they lose power.
Swap drivers between axis to see if the problem follows a driver.
Try moving the step and direction wires from the PC into the drivers away from any possible sources of noise. Wrap them in tin foil and ground it. Use thicker, shielded wire.
Buy better drivers, bigger motors.