It's a problem with the new design for the 20" for both rev C and Intel Macs. Apple however, will not accept that there's a problem.
https://forums.macrumors.com/threads/175145/
http://discussions.apple.com/thread.jspa?messageID=1583047#1583047
They may be of interest.
https://forums.macrumors.com/threads/175145/
http://discussions.apple.com/thread.jspa?messageID=1583047#1583047
They may be of interest.