Need Help! My CNC router gets out of alignment when moving in X and Y at the same time

Hi. I built my CNC router a while ago using a kit from Probotix (180 step motors). I calibrated the motors and if i program a g-code going in X or Y independently everything works perfectly. However, if i want to engrave a picture on wood or mill a small model car, then I loose precision in either the X, Y or Z. I noticed that this happens when the router moves diagonally using X-Y, or X-Z, or Y-Z displacement. I tried reducing the accelerations but that didnt work.

Can anyone tell me how to solve this problem?

Thanks