I spent 2 hours on the phone with Adobe Acrobat support. They had me try every possible option under the sun and we were not able to resolve this issue. While on the phone, I realize that the PNG renders poor only if the graphic has transparency enabled. I asked the support technician to raise the ticket. He put me on hold and returned to say that it was raised, but they declared it a "bug" and suggested I upgrade to Acrobat 9. I did, but it has the same issue.