First thing; Make sure the flybar isn't binding; it needs to bounce up and down with complete freedom.

I bet it's as simple as the upper blade tracking is WAY off. If You replaced the upper head, then You almost always need to adjust the link from the upper blade to the head until the blades at the top stop vibrating. Rev it up and see that the tips of the outer blades track on or near the same line. If not; it'll cause the problems You're describing. Turn the little ball link one full turn either direction and see if it gets better or worse. If better, than adjust the same direction by half turns until it's pretty much on track. If worse, go back the other direction 2 turns, and half turns after that.

Gary