I have run into this a few times with my 1640s as well.
I have an older TDK external burner that won't read at max unless you keep the tray button depressed for a few seconds while closing the tray. This is a built in speed control for noise reduction during playback. It occasionally exhibits the same trait, of not reaching full speed with some titles.
Is this a form of ripguard, or something else written to the disk causing this?