The Pioneer C-302D DVD Player had played all my DVD-R discs in the past with no problem. Starting since last week, I noticed some pixellation on some backup video DVDs and for some of them, it won't even play. All retail DVDs are still playing perfectly, no problem at all. Only some Ritek DVD-R discs that I burned and viewed in the past have problems now. What happened? I bought a lens cleaning disc at Radioshack and it didn't help.

Please advise,
Thanks in advance,
Lucas