Robert has good advice. The big question is: Is it sudden or gradual shift. If sudden it's probably the pulley on the motor. If the shift is X direction it's therefore the X motor. The second most likely pulley is the other pulley on the short belt.
It's tricky to get at the motor pulley - you have to first remove the right rear metal cover. It's held on by one screw.
Then you might be able to tighten it without removing the 4 screws holding the motor in place. Mark the shaft and pulley while you have it apart. And tighten the hell out of that screw. The steel wrench should twist.
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I can't be sure without seeing pictures first but it sounds like you're seeing a sudden shift in the X-direction? Usually this is due to pulleys not being secured well enough to the shaft. There are two small set screws on each pulley, make sure these are nice and tight (probably tighter than you think they should be). To make sure this is the issue you can put a mark on the pulley and a matching one on the axis, then, after a print fails inspect your marks and see which pulley it is that is having problems.
If you're seeing a gradual shift (so that it looks like the print is leaning) it's more likely friction causing trouble. Usually it's either the belt of the pulley making contact with the side of the machine. If this is the case you can undo the pulley on the motor and move it closer to the motor (while still leaving a tiny gap) so that it is no longer rubbing against the wall.
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