You might try a little lower temp for pla.
Also make sure you have a fan running to cool the layers.
When you are doing test cubes the speed slows way down and if you dont
Get the layer cool you can have problems.
http://www.thingiverse.com/thing:18271
Is a good test model when printed at about 55 to 65mm tall.
And it doesnt use much material.
Keep me updated.
Good luck