This is extremely common. It's your Y axis only. X axis seems fine. It's usually the pulleys as nallath says - most often the motor pulley or the other pulley on the short belt - tighten the hell out of these.
HOWEVER it could instead be friction. Push the head around in X and Y. Feel the difference. Oil the 4 rods in upper corners. Wipe all dust off all rods. Don't oil the 2 rods through the head but still you can use oil or wd40 to clean those (put something on the bed to keep it from getting oil dripped on it).
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nallath 1,124
Re-thighten your pulleys. One of em is probably loose.
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