I guess that all your belts are tight and bearing mount all ok?
Is the x axis parallel ie it touches the two front pulleys equally?
Did you change the original speed parameter in repetier?
Hope this help.
If your x-belt is too tight, it will move further in the x-direction than the firmware expects.
And on the flip-side, if the y-belts are too loose, it will move less distance in the y-direction than the firmware expects.
When you put bearings in your x-belt idler, if you got it crooked at all it can lead to the belt constantly tightening and loosening.
Any of these can lead to out-of-round circles. Do another test print, but this time mark the front so you can tell exactly what angle the error is happening at.
Also take measurements. If you printed a 20mm diameter circle, see which direction is short (or long)
Also make sure to calibrate both X & Y.