Is this the first time that you have been trying "3dcarving"? There is a fairly common problem where the pulse polarity on the step signal is reversed. The effect is to lose one step every time the direction of that axis changes. If you are doing a lot of fairly simple profile cutting, this will often not show up as a few microsteps here or there doesn't make a big difference. However, once you get to "carving" type operations with lots of reversals, these lost steps add up. The fix is simple - somewhere in the config setup you will probably be able to flip the step signal polarity - and the effect is magical! Well, it was when this happened on my own machine. Muzzer's test will show up this problem.

However, if you have been doing the same kinds of things before without problems, then listen to the other guys!