Doesn't sound like a crack, sounds like a bad connection.
Check the connection on the LCD itself. There is black tape covering it. Take the tape off, clean out that connection, then put it back together. BE VERY CAREFUL AND DO THIS WITH THE POWER OFF. iPad 2s love to blow filters if you mess with the LCD while its on. Don't use any metal tools, you don't need any tools to get that connector off and on.
I know you have already checked the connection, but that's where the problem has to be. Clean out the connector on the mainboard and the connector on the LCD with alcohol. Blow it out really well. Something isn't making a solid connection and its making the Pink Floyd effects.