I was also going to mention getting a DVD laser lens cleaning disk. Some DVD's just aren't as quality as others so I was having the same problems as you mention with them and once I started running the cleaner have never had a problem again. Can't remember for certain but I think mine was about $14.00.
Mt first thought is to ask when the last time you cleaned it was? If, like most people, you have never cleaned it go to Radio Shack or some other elctronics type store and get a cleaning disc.
My other thought was the discs that don't play - are they burned DVDs? Or are they from a particular manufacturer? Some machines get picky about how a disc is produced. If the problems happen sprodically with all kinds of different DVDs or happens with a DVD that it had once played just fine then it is a defect in the machine. At least that would be my guess. I am not a DVD player tech.
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