ZXR,
One thing, draw a straight line of a length you can measure with a dial caliper do so in each direction x and y. This will tell you if you stepper motors are set to the correct steps/rev. If you are experiencing errors on all axes then I strongly suggest you look at the square of your CNC. If for any reason it's not square you'll never fix the symptom. One good test is to cut or draw a square of equal sides say 10" by 10" if your cnc can handle that size. Then measure all sides they should be within specs. Another way is to cut 4 squares whose sides can be measured by a dial caliper. Let's say 4" by 4". These sides should be almost exactly the same for a small 4" square. Cut or draw if you have an accurate pen mount, these 4 squares at each extreme corner of the cnc. This will test for squareness. Additionally, if you're driving the steppers to fast or too slow you'll get errors. I know this is a lot, but eliminating each possible cause help you narrow down the cause. Sadly my cnc was poorly designed and would not stay square. I also had flex in the spindle mount, there is no substitute for metal in an cnc frame trust me on this.