If you purchase your grblShield pre-modified with the Shapeoko mod be sure to verify that it was done correctly. I just tested mine last night and after setting the parameters to the Shapeoko defaults (using appropriate steppers) my Z axis wouldn't move with out cranking up the current and then it was starting, then stopping with a consistent rythym which based upon the smell and temp was due to overheating.
I changed the setting so all axis were at the same steps/mm and the shutdown issue went away and the steppers were all moving at the same speed. Except they shouldn't have been as the Z axis should have been at 2x microstepping (vs 8x) and at the same steps/mm should have been moving faster. So I pulled out my multimeter and verified the Shapeoko mod. There was a divot where it was modified, but the vias still had continuity. A quick scrape with the end of my multimeter probe and I was good to go.
Just a quick note in case someone is having the same problem.
It is amazing how entertained I was just running three loose stepper motors back and forth :-)