my 2 1/2 year old G4 iMac will no longer read DVD movies or music cds. when a disc is inserted, it spins the disc around for maybe 10 seconds, makes a few quiet, quick 'purring' sounds then ejects the disc. however, it will still recognize and respond a blank disc. for last little while it would only do this if the computer had been on for a long time but now it does it even after a good night's rest. any ideas or help?