When attempting to sculpt on my current project, the center of the mesh distorts, as seen below.
Attempting to use the crease tool on the mesh does nothing UNTIL I get to the center, where the above change takes place. I tried several other tools and I get the same (or similar) results. Let it be noted, too, that I'm using a mirror modifier, and that center seam is where both sides meet, in case that's relevant.
Here is the .blend file, if it helps.