I have Innovative 75A steel engine/trans and Hasport billet aluminum 88a diff mounts. Power transmission is very direct and predictable but I get a bit of back and forth movement of my powertrain, as if it is held in place by stiff springs with no damping. Has anyone noticed this with a full mount setup?

 

I have the same. And only a little. You may have a loose bolt.

 

Mine is also minimal and only when I try to modulate the throttle during daily driving. Flooring it solves the problem.