I attached an image of the Touch Screen test procedure. You might want to start there. If it shows that you have problems with your digitizer, the manual recommends that you " Check:
Inspect the touch screen of the Cover Front assy.
1. If dirty – clean it
2. If the touch screen is damaged – replace the Cover Front assy.
Inspect the ZIF connector of the Cover Front assy to the Main PBA.
1. If not properly connected – disconnect and reconnect
2. If dirty or oxidized – clean the ZIF connector.
Inspect the flex film for the touch screen on the Cover Front assy.
1. If the flex film for the touch screen is damaged – replace the Cover Front assy.
Now if it is malfunctioning, you can/should replace the digitizer to remedy that. The original part number for the Cover Front assy White is 1239-3969. It is available at places like this. Now if you need the parts manual as well as the service manual with embedded videos about your X8 it is available for your download right here. :-) Good Luck