Hi Paul,
Best guess, without seeing it would be hardware !!
If you have the original Mac OS Disks that came with it, boot up from one and have a look at the screen, this will rule out software, if the screen is okay whilst booting up from the OS install cd's then you will have to do a Software reload...
...but like I said earlier I would imagine that this is hardware, is there a story to the screen getting a stripe...or did it just 'happen', was it dropped ?
regards
Ric