I would switch to an ER70S-3 wire in lieu of the -6 wire. The -3 wire is lower in Si which may be your problem. The type of cracking you are talking about is associated with transverse shrinkage stresses during solidification of the weld metal, and often takes place in alloys rich in low melting point impurities, or low ductile elements such as silicon combinations.

Typically, these constituents are low melting point constituents such as Sulfur, Phosphorus, and in this case -I believe-low ductile silicon combinations.  The low melting point constituents, and/or low ductile silicon combinations are typically left along the grain boundaries as these are the last parts of the weld to solidify.  As the weld solidifies and starts to cool down, transverse shrinkage stresses are introduced, the grain boundaries with the layer of low melting point constituent and/or low ductile silicon combinations (usually situated in the center of the weld) can no longer take the stress and separate from each other creating a crack along the center line of the weld.

In addition to changing to a -3 wire you may want to entertain changing gas as well. You might try a 85%Ar-15% Co2 or an 90%Ar-10%Co2. This will create a shallower weld penetration and produce a more parabolic weld bead profile. This would address the concern of the depth to width ratio concern that was mentioned in a previous post by pipewelder-1999

Hope this helps.

Mark M