it seems like a hardware failure.
does that happen in the console also? try not to log into X and rather log in to the console and try the command :
then try to copy and paste some lines that will fit into more than one screen and try to scroll using the scroll keys to find is the problem related to some X configs, or is it a general problem.
Try to scroll in Wndows, does it scroll in windows? if not, it is more likely a defective keyboard, and you will need to replace it, if it do scroll in windows, then try to play with linux keyboard setting in X.