vertices jump about when dragging I am retopologising a mesh using shrink wrap modifier, but some of the vertices jump about when being dragged for alignment:

For example if I press G and drag the vertex below to the right a bit it, it will jump to the right hand end of the mesh.

Any ideas what is going on here,

Regards



Thanks for your responses hawkenfox / cegaton,

I have looked further into your suggestions.
As requested more info in screen shot with 3DView status bar.


Note: "Snap during transformation" NOT enabled.
"Type of element to snap to" is Increment, also tried "Face" as you suggested but no luck.

I am new to the shrinkwrap modifier and its settings. 
I'm not sure what or where "Snap onto Self" and
"Project Individual Elements on the surface of other objects" are.
Here are the Modifier settings:



I have used this method on another mesh without any problems and only certain vertices on this mesh seem to be a problem.

I hope this helps

Regards



Yes, the shrinkwrap tool is used with the "bsurface" and grease pencil tool to speed up the retopology process. ;o)
You must have set your "Type of element to snap" to vertex or segment and have your "Snap onto Self" turned on. Check that Type of element to snap to is set to face. You can leave "Snap onto Self" on and turn on "Project Individual Elements on the surface of other objects"