I have acquired a used NAD 512 DVD player (which is now hooked up to a NAD 762 receiver) and most of the time it plays beautifully. But occasionally, 1. when playing a DVD movie - usually after the first half - it starts skipping parts, like jumping 5 secs ahead, then 7 secs, then 3, etc. The DVD itself is faultless. 2. when playing music CDs sometimes at the beginning of a new track - never in the middle of it - the sound is suddenly dead but the CD continues to spin. If I eject then reinsert and skip to the track in question, it plays OK. Both faults occur at random, I can not reproduce them at will. Is there anything I can do about these or do they require a major overhaul? Can anybody help?