Y axis is not working right and shifting mid print

[matc94 0]

so I've printed 3 things now successfully and I was printing something else it was nearly done about 4 hours into it and then it started screwing up and shifted over about .75% of an inch not sure what is causing it I noticed if I turn off the machine completely and then try to manually move the head around I can move the head around the x axis but when I try to move it around the y axis it locks up in certain places.

[IRobertI 448]

You probably have a loose pulley, that's the most common cause:

http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#shifted