Currently Being ModeratedNov 13, 2012 9:35 AM (in response to Nikifamily)
I would suggest testing with a different brand of DVDs as some of them are not very good.
If that doesn't do it, you are probably looking at hardware service. Contact Apple using www.apple.com/support/contact if you are in a country that supports it or call the appropriate phone number on this page. Alternatively carry the iMac in to an Apple retail store or service provider to ask for an estimate.