I don't know what you're doing, it doesn't really sound like how I did it. (EDIT: It sounds like you just inserted a digger, so I guess it was similar.)
It uses two miner glitches, and one is the old one. The other one is that miners can mine in mid air for a few frames (maybe not a glitch, but still weird). The whole thing requires two pixel precise moves per iteration, and uses only builders and miners. So it goes like this:
mine-pause \ break \ unpause-build
A dash means the two actions were in adjacent frames. I realize that it could be done click-F7-click, but IMO it's easier when there is a break.