Shoots out code U1, freezes up after abo...

When I first turn on my Pioneer Elite CLD-59, push play and the unit makes a few groaning sounds, the tray opens up and the error code "U1" comes up. I hit play again, and the unit plays the LD. After playing for a while(1 or 2 Sides), the picture starts to freeze up and sometimes 7 or 8 chapters are lost and the unit wile start to play at a new chapter. I've checked my discs and this happens on virtually every LD.