We need much more details than that....Is any portion of the library on an external data storage device?
Is the problem when any external storage device is hooked up, and if so by what means?
Is the problem when the hard drive is over 85% full?
Is the problem when Time Machine is running?
Is the problem when Spotlight is indexing?
Are you using Home Sharing to another device such as an AppleTV?
Does it respond to command-option-escape keyboard sequence?
Do you ever get a four language screen telling you to restart your computer?