Poor Print Quality

I have been working on getting my printer up and running and I have finally had some success. The prints are very poor quality, usually around 2mm up on the z-axis it begins to drift to one side, but only sometimes. I have (semi) successfully printed an iphone case, a bracelet and 2 20mm cubes, but recently when I go to print other things it start to drift to one side until 3-4 layers later it is printing in mid air. The weird thing is that this only happens on some prints, and I am stumped as to what needs to be adjusted. Any help is much appreciated.
Pictures of failed prints:

This is probably your x (or y, can't tell from photos) axis losing steps. Adjust the trimpot on the axis controller, and see if that improves things. If your controller is too hot to touch and the axis has jerky movement, go counter-clockwise (less power). If it seems fine, but loses steps, go clockwise, and make sure your controller doesn't overheat with the extra power it has.

You've likely got much else that needs calibration, too, but that's part of the fun. You're right to concentrate on this major issue first.

Also make sure your pulley set screws are tight and your belt is properly tensioned. The pulley could be slipping on the stepper shaft. Or if your belt tension isn't tight enough, the belt could be jumping teeth.

You might also to make sure there's no binding in your x and y axes. With your motors off, slide the x-carraige (and then the bed) back and forth the full length, it should feel smooth with the same resistance along the entire path.

I had a problem on my first printer where the bed would get tight and bind at the outer extremes, turned out is was improperly aligned bearings.

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