THe same files work just fine when I transfer them onto my pc.
I've tried attaching the pc's monitor to this laptop and then toggling between the two screens while I play the files and they're then screwed up on both screens (which I think suggests it's not the LCD doesn't it???)
I've asked Dell support but their suggestion is that I back up everything and restore the machine to factory settings and see if it goes away.
Seems a bit of a scary and drastic course of action to me, so I just wondered if anyone has any thoughts or ideas I might try out first.
Thanks in anticipation of any helpful suggestions...