If the problem is, in fact, traced back to the ATI graphics cards, it won't be the first time that AMD's hardware has been blamed for iMac troubles. More than two years ago, Apple began investigating with the then-new iMac line that sported ATI graphics cards. Several weeks later, Apple issued a that seemed to solve the problem for most users.

This year's new iMacs have also been plagued with performance issues. Within days of their Oct. 20 introduction, users complained about of Flash video. A Nov. 9 targeted the problem, which was resolved for some, but not all, users.

has not responded to multiple e-mails asking for comment on the 27-in. iMac screen issues, including one Friday requesting information about the two-week shipping delay. Apple typically refuses to comment on hardware issues, preferring instead to quietly publish a support document to its site.