Mesh sculpt problem

I just did some quick freehand sculpt using mesh sculpt script from 2.41, here’s what I have found on the problem.

The accuracy of the point of sculpting is unpredictable, i.e sometime, it does not response to the mouse and the sculpting area is not directly underneath the cursor.

The mirror half split up if you sculpt on the mirror axis, relax the area does not completely fix the problem.

Sometime the mesh completely disappear and re-appear as well.

Is anyone having similar problem??