Some CD's won't play... what's wrong?

Hi all

My Krell 300CD player has trouble playing some CD's which used to play perfectly before. You can hear the drive trying to read the disk, but then it gives up. The CD's that can't play are in good condition, some even brand new. I can sometimes 'force' the player to read an obstinate disk by disconnecting at the mains and re-connecting, or opening and closing the drawer a few times. Certain CD's play fine every time.

Is it time to replace the drive mechanism, or can the laser be re-aligned or something?
Kind regards
Mick