I think this is resolved. Jim Neeb had a side comment in one of this laser video's that he solved the z drift by changing the z step to active low to resolve the issue. This change seems to have fixed the drift issue for me. I have not gone back to see if changing the look ahead  lines resets the dro's.

Thanks
Mark

Not sure it is relevant, but I don't think G70 is supported by a Mach4 mill