Hi Paul
The obvious answer is it looks like you were missing steps. The fact the router dug deeper indicates it was when Z was lifting so by adding the counterbalance you have solved the problem.
Vcarve has a lot of Z movement and it will test the Z axis in any machine.
While your fix has worked the alternative is to re-tune the motors to eliminate the missed steps.
I like the pnuematic device idea - can you post picture?